Seems like you might be right. My gut says you may be.
I looked at the ILSAC specs:
They don't mention TBN by name.
A chart for API SL I found also fails to mention TBN:
It seems like you might be right, but we should compare apples to apples.
I don't have time right now, but a good check would be to look at VOA from 2003 for a given oil, then compare VOA from 2006.
If the TBN went down in, let's say, the first five cases we researched, you might be on to something.
Alernatively, maybe we're doing more dino UOA and less fancy-pants oil UOA?
If all we test is the old-school Mobil 1 SL, German Castrol and Amsoil Series 3000, we'll certainly get higher TBNs than if we test the conventional 5W30 varieties of Pennzoil, Valvoline and Exxon Superflo.
I appears oils not meant for diesel use or extended drains get virgin TBN betweeen 6.5 to 8 while oils meant for diesel or extended drains run from 11 to 13.
Or, maybe we're seeing more long-interval oil changes, with greater depletion than we had previously?