[pmarreck]

This is because iMessage (somewhat notoriously) allows not just multiple single identifiers (such as a phone number or email address) but multiple identifiers AT ONCE (such as BOTH a phone number AND an email address)... And all of these are treated differently!

iMessages from the same person will end up in 3 different conversations based on whether they specified (for that conversation) only their cell number as the recipient identifier, only their email address as the recipient identifier, OR BOTH! Which is of course madness. Which is why I tell everyone within earshot to ONLY check off the cell number identifier (even on their non-cell devices) and uncheck ALL other identifiers, for sanity. (This is under iCloud settings somewhere.)

But that explains this. Why is it like this? Well, once upon a time there was the iPod Touch, which had iMessage but didn't have a cell connection or cell radio or cell number (think: kid with an iPod Touch who never had a cell number). Also, Macs have iMessage and don't have those either.