Lines of descent

The impression I had always gotten was that Intel was like Disney Animation, with two fairly separate teams doing interleaved products, separated in time by one-half a product development cycle. Given that, you would expect the 1st, third, ... products to have more in common with each other than they do with the 2nd, 4th, etc.

There are solid product-management reasons for this, but also a fair probability that if you are, e.g. a fan of the odd-numbered products you will find something to not like about the evens. That said, I, too, am a fan of the PII (aka Sexium/P6, IIRC) based line. Among Intel CPUs. :-)