There are "slight" production differences in the shades, but these are more of a batch-related thing rather than a facory option. Also, manufacturers have a slight difference, e.g. Triplex versus Sekurit, etc...

You should not really be able to notice a difference though unless you look very close.

If you want to double-check any windows you have, put a clean sheet of A4 paper behind each one and that will highlight any difference immediately.