White generator is grey online

Well i've a very strange effect.

I have a video where the background is white, completly white. I crop the video and i put a white bright background.

So in FCPX i see a perfect white uniform background.

If i watch the final result on my Mac, it's withe !

when i upload the movie on plateforme like Vimeo, Youtube or Look.at OR if i watch to the movie on a PC. the white generator is grey !

i've tested using : White-White Bright, using Grey in 0% and whith a degrade from 255 to 255. they are all grey !?