No, I've figured out that the distinction is that you add an addressee at the top, which violates the "one and only one" condition on the collapsing.
[me: looks at the topic on web again] Ah, or perhaps because it is no longer at the "top" -- my and your greetings are.
In a couple of groups in which I'm a mod, I've actually chided people for including the whole prior message, complete with signature, in their responses, because of the unreadability situation this causes.
I generally agree, especially the bit about quoting the whole prior message and not trimming it to the relevant part. And the other person's sig is (almost?) never a relevant part.