According to Why are trivial edits discouraged?:
Edits made by the same editor in a short period of time are collapsed: no matter how many times a post is edited within a 5-minute window, only one revision is stored ...
In this recent occurrence, revisions 2 and 3 (by me) for the question are less than 2 minutes apart, and yet they weren't rolled into one revision.
I posted an answer between the edits, but it's not the first time I've done that. Shouldn't the timer be post-specific?