Fifteen isn't the maximum for MPEG2, and currently some terrestrial Digital Video Broadcasts like Freeview use a GOP size of 40. Incidentally this makes for considerable problems when trying to transfer recordings to DVD.
The I-pictures are the only ones directly derived from the original image. The other B- and P-pictures are synthesized with reference to that. The DVD standard says you can only have 15 altogether in one GOP: one I-picture and fourteen B- and P- derivations.
It's not my understanding that an I-picture has to start at a scene change, and may only be achieved by a 2-pass encoder which can store references to such events and thus optimise the whole encoding.
So... Why is my Sony HX-900 only drifting by an average of 5 or 6 frames after an A-B edit on DVD+? The worst case edit was 11 frames out but that only happened once. Another strange happening is the accuracy at the "B" end of the edit. Unlike the "A" end, it is sometimes only a frame or so adrift from the intended point.
I wonder... Is the Sony capable of inserting a new I-frame in an edit? On the HDD an edit has to be at least 50 frames duration unless it is the very beginning or very end of a title. The only exception is editing over a previous edit where you can knock out just 1 or 2 frames over the "splice" area.