Now with extended BT, the diver is usually limited by available gas and comes up with BT remaining, hence the nitrogen load is LESS than maximum.
He can't use all the extra bottom time, so he ends with less nitrogen on a longer dive.
I see lots of people run out of BT on air. Not on nitrox.
Weird... with a RMV of 0.4 cu ft/min, using 32% nitrox in a regular ol' alum 80, starting pressure of 3000 psi, dive to 100 feet... the NDL would be 30 minutes (from the PADI table; most computers will give something similar) a diver would use 48 cu ft of gas, which means they would have 1200 psi when their remaining bottom time hits zero.
My experience has been that it's perfectly possible to use "all the extra bottom time" while on nitrox. Just a matter of planning the dive.