There is a 5-minute grace period after the question (or answer) was initially posted. During this period edits (by the same user) do not show up in post history; they are not even stored on the SE servers.
Same period applies after edits. If you edited a post, you can edit it again within 5 minutes without generating a new revision; the previous one simply gets replaced.
In order to prevent a series of tiny edits from showing up in the revision history, a single user who edits a post is given a 5-minute grace period. During this period, any additional edits they make are collapsed into the same edit in the revision history, displaying only the final outcome of all their edits within that 5 minute period. The grace period ends after exactly 5 minutes, but also ends if another user edits the post, at which time their grace period begins.
(Demo: this was added during the grade period.) The 5-minute period does not renew with subsequent edits. For example: if you edited a post at 12:00, 12:02, 12:04, 12:06, and 12:08, the revision history will keep 12:04 version and 12:08 version. Here the 12:06 edit is outside the original grace period, so it creates an entry in Post History table. It also begins another 5-minute period.