Math Confusion

i can't figure 1 out save life. document size 2592x3888. following code:

w=app.activedocument.width;
h=app.activedocument.height;

alert(1000-w);
alert(w-1000);
alert(1000-h);
alert(h-1000);

the alerts come out as

1592

2888

the first , third alerts should come out negative. don't. if change code w w/4 , h h/4, alerts follows:

-352

-27.9999999999998

not should first , third come out positive, last 2 should whole numbers.

what missing?

i think results depend on version of photoshop using , ruler unit settings.

if using cs4 , want w , h values in pixels this

w=app.activedocument.width.as('px');
h=app.activedocument.height.as('px');

alert(1000-w);
alert(w-1000);
alert(1000-h);
alert(h-1000)

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