Duplicating a channel uses an unexpected channel number

I currently have 15 movie channels as 10001 through 10015, and then for "future proofing" I started numbering my TV channels at 11000. (I can't remember if I chose 10000 as the start or Channels did, but it's not relevant here.)

If I duplicate a TV channel at 11000, it creates the duplicate TV channel at 10026 for some reason, making it appear before the channel I'm duplicating.

My expected behavior would be to take the number of the channel being duplicated, and then increment it until it finds the next "free" slot.