Historic background to the two systems
Correct me I'm wrong but I think a little discussion of the background to the two different systems goes right back to the original 100/200 series. The pack system on the 3/5/600's is substantially the same (albeit improved) design and part of that goes right back to the old 727! Once Boeing came up with the idea of extending the 300 into the 400 they realised that the original 2 zone system was not going to work. They based the three zone system on that used on the 757. This system has since been updated and used on the 800 and 900 plus BBJ2.