Happens here too, and I thought maybe it's a problem with the drive at first. But after doing PI/PO, BetaJitter, etc. scans on a Plextor 716A, discs do verify and test perfectly fine (superb PI/PO results actually), so I figure it's just the way it is -- the laser doing auto calibration as it goes, so it's varying the laser power depending on how far in the burn it's gotten (ie. how fast the disc is spinning), to create an optimal burn.
If the laser power is kept the same all the way through (ie. no coloration changes), then you may not get an optimal burn at the end of the burn (which you often see on other burners).
Anyways, see my :: Good & Bad Media for Pioneer 109/A09 :: thread for my PI/PO scans and you'll see what I mean -- excellent scans, but they do have noticable rings on some discs (depends on burn speed & disc).