A linear view has them peeling off from the others in some sort of sequence. A more realistic view has them peeling off from each other, in part, and coming back together, in part, at different times.
Think of horses, donkeys and zebras.
Pretty obvious that they all share a very similar body plan, but what's the truth of their relationships ~ what does their DNA say?
Remember, donkeys and horses can currently reproduce with each other. Both can still breed with zebras. Yet, there are three Species of Zebra. All inhabit different areas, all have different chromosome counts.
Then, there's the "Ultimate Peoples Hero Horse" the Hippopotamus. They don't mix well with any of these guys! Yet, they have the body.
No, because you have hidden assumptions in there. It's not a matter of basic logic. You are assuming things about a linear relationship between genetic patterns and body plan that may not be true. Genetics is quite a bit more complex. Some very different genetic blueprints could create similar plans, and similar blueprints could create very different plans.