EDIT: It seems that I've also had edits that stacked well, so I'm not sure if I had a timing problem, or was just delusional. The one thing that can be fixed, that I'll turn this question into, is a feature request so that when I go to re-edit (either within the 5-minute grace period, or before a suggested edit is approved), it brings up the old edit summary text. Right now, it brings up a blank edit summary, and this replaced the first one I did. This isn't really intuitive, and even knowing that's what it's doing, I still forget to put everything in.
It has happened a few times now, when on a site where I don't have edit privileges, that I make an edit. I then see that the edit didn't work quite right, so I go back in to re-edit it. This creates a brand new edit review, with an additional useless summary.
How about we have the same grace period to re-edit your up-for-review edit, as there is for a normal edit?
Of course, it wouldn't be possible once it's been approved.