I B B P B B P B B P B B P B B -- Why 2 B's

This is just bothering me -- haven't found an explanation yet. I have noticed that many/all of the/my MPEG files contain a GOP of 15 frames and the 15 frames go like this: I B B P B B P B B P B B P B B .

I did read an explanation on How Stuff Works that explains that the B (bidirectional) frames are interpolated from surrounding I frames or P frames -- which I read to mean a B frame had to be surrounded by either an I & P frame or a P & P frame.

So where did the 2 B frames come from. Does the hardware/software take the surrounding P and/or I frame and interpolate 2 frames.

And is this 15 frame GOP a standard. Can it be changed on the fly, i.e. within the MPEG-2 file. I can see how PC software can handle this, but what about the hardware DVD players.

Thanks.