Gmail threading is broken. It seems to work on the basis of subject line, and some unknown-to-me timing function.
Examples: I commented on two different Facebook groups. I am setup to get replies to further comments. The replies from both groups were intermingled.
Recently I sent out notices to customers that their special orders had come in. Each message was sent individually, but they were sent over a short period of time, and the subject lines were indentical.
So Gmail threading is broken.
Normally when I do this, I want to split out their replies to add to their Streak box. (very cool CRM that integrates well with Gmail) However I cannot add a conversation to more than one box.