Why ctrl+a > delete doesn't delete all -- I think I understand

I often make one long recording and then break it up into single song projects.
 
I copy the last song I did, rename the files, open the project and delete all the audio and envelopes.
 
For the longest time, I've puzzled over why I often have to do this twice. The first time I do ctrl+a > delete it deletes a bunch of tracks and envelopes but inevitably leaves a few. Then I do it again and all are deleted.
 
I have always thought this was a bug.
 
Just now, doing it in Platinum, I noticed that not all the tracks had clips set as the edit filter. After executing the delete the first time, all the tracks with remaining audio have their track view edit filter altered from the envelope they were previously set to to clips. Now, doing it again deletes all (audio and envelopes).
 
So, curious, I did undo and put all the clips back and then did ctrl+a and set the track view edit filter to clips and then hit delete and it all disappeared.
 
I'm probably the only one who hasn't understood that the edit filter on the tracks was altering the scope of the delete, but I thought I'd post here in case I'm not.

Thanks, Swamptooth. Subtle. I'd never noticed it before.
 
I feel like a goldfish rediscovering the castle anew with every lap around the bowl. 'Oooo cool castle!'
 
The other one I happened into the other day was the Revert menu item. When restoring, I always go to the directory and make copies of the files so I can roll forwards and backwards and so on. Do you happen to know if Revert is covered by undo?

When I found Revert, I went back as far as X2 (oldest still installed) and validated it's been around a while.
 
When Reverting, I usually REALLY care about a safe recovery though, so I'd have to really understand exactly how it works in order to trust it when it matters.