Recently, I edited one of my answers to update a link to a better resource. I clicked "Save" without entering a description of the edit (edit #1).
Then I realised that the question was asked by a new user who may not be able to easily identify the change that was made. So I edited it again, this time only to add a detailed edit description (edit #2).
Since edit #2 was made during the grace period (within 5 minutes of edit #1), the system has merged them into a single edit, but only the edit comment from edit #1 was recorded in the revision history, whereas edit #2's comment was simply discarded/ignored.
I'd suggest that either the most recent (non-default) edit comment is taken or that the comments are concatenated to provide a more useful edit history