Whats up with this math

bottom = track._height + track._y;
I am really out of it today so this is prolly a dumb question…I have a variable ActionScript:
bottom = track.[color=#000084]_height[/color] + track.[color=#000084]_y[/color];

track._height is 170 and track._y is -169, so when i add them so, why does it give me the answer 87.5 when i trace(bottom); thanks