Wire amps question?

In some installations a wire temp of 90 deg C is allowable such as power distribution on a telephone pole. This would not be good in a wire chase indoors or inside a plastic pipe.

%0 degrees C is the maximum where someone could grab the wire with their hand.

The table shows this: max allowable current for max temperature.

The "temperature" referred to is the "temperature rise" from the conductor. 

The more current that passes through a conductor, the more temperature rise. 

The ability of the insulation on the wire to tolerate the temperature rise becomes important.

You are mis-understood the chart meaning under temperature column. Take the first row data( 4/0 wire size) as an example, as current passes up to 195A, wire is heated up till 60 C. As current passes up to 260A, wire is heated up till 90 C.

This factor is designed to choose the wire quality as it is marked on the wire jacket. Indoor wire choose 90 C type can stand as high as 260A.

read the heading again. The numbers are all at 20ºC, the different current ratings are for different types of insulation, which are rated at different temperatures. 

So #12 is rated at 20 amps at 20ºC for the poorer insulation and at 30 amps at 20ºC when better insulation is used.

The maximum current rating for a wire is based upon how hot it's likely to get.  If it's already in a hot environment, it's going to get hot quicker with a given amount of current.  The rating is about safety rather than what it's capable of carrying.