brush bleeds over selection

Because you have zoomed so far into the image.  You are so far in we can see the pixel grid! View at 100% and there'll be no bleed visible.

The piece of the puzzle you're missing is that selections aren't necessarily either/or. Selections can be any percentage between 0 and 100% selected.

The selection edge, the "marching ants", isn't a solid limit. It just marks pixels that are 50% selected or more. So it can still be selected outside the line, just less than 50%. However, when you add 40% to 40%, it builds up. Do it many times, and it approaches 100% selected.

Probably because you created the Selection that way. 

Or maybe you misunderstand what a Selection does exactly; if a pixel is selected with X% then painting there will lay down accordingly, but repeated painting will accumulate. 

Can you go to QuickMask Mode, save and provide the file.