Windows discrepancy (but?) when using renderItem.timeSpanDuration

I'm using renderItem.timeSpanDuration = comp.frameDuration;  in a script that  renders out still images from comps.  works fabulous in OSX.
But in windows, something wonky is occuring, for example when it's a 25fps comp,  it applies a value, such as 0.04000000, it accepts the value, but when I have the script check what value is stored,  the value is changed to 0.040006514! resulting in a duration of 0002 (rendering out two frames),   sometimes it causes it to render a frame too soon, or not, reslts vary depending on comp frame rates.  Havent figured out the logic.  But on Mac OSX, if it applies 0.04000000, it reads back a perfect 0.04000000,.
I attemped to alter the script to round out the number, but no luck.  I thought perhaps if I entered a value lower than 0.04 it might be counter the offset, but AE didn't accept that. 
since this does not occur on Mac, I'm guessing it might be a bug in AE or some windows anomoly?
any ideas? thanks.