Why is the ground on top?

This isn't spelled out in the NEC. It may be a local jurisdiction thing. There are two reasons that I know of: If the ground is down and something hits the cord it will be the last thing to come out of the receptacle. If the ground is up and something like a skinny metal piece falls on top of the cord it could land so that it crossed both prongs on the plug causing a problem.

Personnaly, I prefer the ground down.

Not covered in the NEC book, Many believe that as you pull out the plug and something metal were to fall on the plug it will hit the ground prong first because it is longer and then there's a 50-50 chance it will also touch the common (white)and fall to the floor. The other one would be to hit the ground and feed (black) causing a short. Or hit it and deflect everything and hit he deck doing nada. Been in houses here in New England where all are up and all are down, and a mixture of ups and downs. I would think if the local electrical union came out and said all should be up then you would see a change. I sure would miss that double face looking out at me. Have been doing it this way for over 25 years. If the house has mostly ups I would ask the home owner what their preference was and possibly do it up rather than have to change it later

I have seen a lot of houses where some are up and some are down. I think some electricians are not paying attention when installing outlets. I have even see some switches installed upside down.

Though if all the outlets are this way the electrician must have done it on purpose as others have mentioned.

Up or down is beside the point. The point is that the are working correctly. A simple plug-in tester for correct polarity and grounding is at the top of my priorities! Correct GFCI operation comes second. Thermal breaker age and 'tripping' incidence comes third. My video/audio ,computer/modem equipment can only be properly protected(through polarized surge protectors) if the house wiring is right! Given my investment-you'll understand my concerne!

Mine seem to be on the bottom -- I live near Cleveland, Ohio.