Why does a neck pickup sound different compared to a bridge pickup on a guitar?

Im going to disagree with both of the answerers.

The first reason a neck pickup sounds different is the pickups charachteristics. On both active and passive models, the pickups are made much differently. There is normally less of a magnetic pull from the magnets on the neck position pickups. With typical passive pickups, you will see that neck pickups have a resistance of anywhere between 4 and 10k ohms, and bridge pickups are around 7-14k ohms. Bridge pickups are much "hotter" because the strings elliptical movement is much less. You can see that when you strike the string, it moves much more towards the center of the string than near the bridge. Even in the EMG 81 and 85, the pickups are made much differently. The 81 pickup is actually a rail sensor (under the EMG cover) and the 85 has pole pieces. 81's can also be used in the neck position, but have a much brighter sound in that position, but still sound different than the same pickup being used in the bridge position.

The second reason was touched upon in the first reason. The strings elliptical movement is different towards the middle of the string and carries different tonal charachteristics. You can hear this different by plucking a string with a pick very close to the bridge and very close to the 12th fret.

Different pickups are made for different positions for both the movement of the strings, balance of tone, and the output of the pickup.

As for Lucasmans reasoning of the harmonic node being under the 24th fret... well, its true, there is a harmonic node under the 24th fret. However, that is only in an open position. As you know with harmonics, if you put your finger on the first fret, the harmonic moves from the 12th to the 13th fret. This is true of the 24th fret as well. So, if you played only open strings, I would agree with Lucasman, the 24th fret would be ideal for a pickup. However, that is not the case, so that reasoning does not apply.

That's a really good question. I'm well aware of the differences between tones but never thought of why that is if the pickups are the same. You already answered a portion of your own question with the fact that different pickups are made for different positions. But lots of guitars have the exact same pickups all around. The only thing I can come up with is the difference in the wood tonal characteristics, being closer or further from the neck. Regardless of whether it's the same kind, that pickup should be getting the sound vibrations of the strings from the area it is placed. Being closer to the neck seems to bring a deeper tone while further away brings a sharper tone.

I have custom DiMarzio pickups in my Ibanez and the neck and bride humbuckers are as different as night and day. The neck pickup is very deep and rich while the bride humbucker cuts like a knife. They are both Steve Vai signature pickups but the neck is a Breed and the bridge is an Evolution. Versatility is the reason for wanting different tones out of your pickups. If they all sounded the same there would be no need for different pickups, and there are guitars with one pickup.

You said neck pickup twice. I assume you meant to say that the neck pickup gets a richer, darker tone, while the BRIDGE pickup gets a thinner sound with more 'twang'. I'd totally agree with that statement.

The location of the pickup has a tremendous effect on the tone. Take the exact same pickup and put it in both the bridge and neck position and you'll notice a big difference in sound. Even really small differences in position can have a big impact. That's the primary reason people will say they DON'T like 24 fret instruments. That spot where the 24th fret goes is a killer tone zone. If you've got 24 frets, you can't put a pickup there!

There's a special 24 fret Satriani model from Ibanez where they squeezed a stacked humbucker right up against the end of the fretboard so he could get that 22 fret tone with a 24 fret instrument. Uli Jon Roth's custom Sky guitars actually have pickups underneath the fretboard so he can have his extended range 32-fret guitars without sacrificing tone (of course, the neck pickup is useless above the 22nd fret or so).

Because of these differences in sound, pickups are designed to better enhance the tonal qualities of certain positions. People will sometimes stick pickups marketed as "bridge" pickups in the neck position, and vice versa, and sometimes it sounds interesting, but it usually sounds crappy.

Different Guitar Pickups

All of the previous answers are wrong: the reason why a pickup will sound different depending on the place in the body is because of the string tension: grab a string over the neck pickup and move it up and down. Now do the same thing over the bridge pickup; the difference in tension makes the sound be thin or thick (it will sound the same when playing on different positions unplugged), because when the string gets enough room to vibrate, more harmonic over- and undertones will sound and give the note you're playing a different sound.

well yes they could have been wired differently, there is also the possiblility that the pickups on one guitar were covered, and on the other they were not covered.