I don't know anything about Flex, yet, but in Flash when you create text fields you have the option of left-aligned justification,
center-aligned, right-aligned, and I think also full justification.
While this does not explain the differences you see in platforms it could account for the lack of continuity between your text fields' alignments in both platforms. One thing though, the device fonts might be different between operating systems and you might not have chosen to embed the fonts into the Flash file. If you embed the font the problem should go away. I recently did a Flash project (very small one) where I previewed in OS X and in Windows (Vista, 7) and I got no differences after embedding the fonts.
Hm i explicit set the font , hmm good hint , I keep digging
My guess was different fonts too.
Did you explicitly set the font? Or did you explicitly embed a font?
If you just set it; are you sure that the same font exists on both systems? I thought font handling was radically different in Windows than it was on OSX