IIRC the minimum bbl length to not be an SBR is 16", as you said, but the minimum OAL is 30".
If you look at a preban AR15 with a 16" bbl and a collapsable stock then when it is collapsed it is just over 30"
My Bushmaster Bullpup is just over 30".
I believe the 26" length is for shotguns.
Makes a lot of sense, don't it?
-Mick