Its because the Adobe rendering is doing full-pixel anti-aliasing and the OSX rendering is doing sub-pixel anti-aliasing. ( Read up on clear-type and the like and it should go a good way to explaining the difference )
Considering the context of Photoshop, its probably better that its kept to full-pixel because of the way in which your using the pixels as part of a composited layer image one of top of the other. So the render engine takes the whole pixel colour value from each layer at that position and then generates the final display colour.
If you were to split that into sub-pixels, it would generate a whole lot more work for the display... roughly 3 times the amount of processing.