I've edited the body of this answer (link to the answer's revision history) twice after ~21 minutes I initially posted it, the second edit ~3 minutes after the first edit, but the 5 minutes grace period for those two consecutive edits didn't apply; in fact the revision history shows two different revisions, one ~3 minutes after the other (revision 3 ~3 minutes after revision 2).

What happened to the grace period here?

Revision 2: 2015-11-06 15:02:50:

Revision 3: 2015-11-06 15:05:06: