That's not right.....
The L800 was largely contracted out, it's well known that the N9 was far more "in house" than the L800*, that slowly changed with the 900, & is probably very different for the 920 now.
You have variations in housing quality (which includes colouring/coating among other things), some white housings will look better than others, it's the nature of mass production.
It MAY be possible that "whiter" housings & better glass fronts are being reserved for the L800's etc, but generally production doesn't work like that, too inefficient...
More likely is that because we've had much greater no's of them being produced for much longer now, the probability of having better quality builds** in circulation has become higher.
That's assuming there really is a difference, it's all way too anecdotal thus far, you'd need to compare a large pool before you could come to any sort of rational conclusion.

*This was done because Nokia didn't have the capacity for Qualcomm related tech at the time, so in order to get to market quicker, most of the work was contracted out.
Because of the specialised (stop-gap) design of the L800, that will remain the same for the full term of it's manufacturing life...
**at least in SOME aspects....