This is standard operating procedure. 8800GTX was 90nm. 8600/8500/8400 were 80nm. Nvidia eventually re-released their tweaked high-end GPU at 65nm, the 8800GT.
ATI does the same. The HD 2900XT was 80nm, 2400/2600 cards were 65nm. And also just like Nvidia, ATI re-released their tweaked high-end card on a 55nm process.
I may be wrong but I believe it is done for cost reasons, the lower/midrange parts have a much smaller profit margin, so the smaller the physical GPU size the higher their profit to offset this. The second, and likely just as important reason is it also will give the fab facilities and GPU companies time to work the kinks out of the smaller node process size, using the simpler lowend/midrange cores. Once they have the bugs worked out they can use the process size on much larger and more complex cores, as they have both done with G92 and RV670.