Rsync to-check grows during backup

Question

I am currently transferring about 40 Gigs of files from an external disk to my hard drive, and the rsync progress output always shows something like

... to-check=1100/24094)
... to-check=1099/24094)

But during the transfers, the second number grows, and the first never goes under 1000.

... to-check=1200/27199)
... to-check=1199/27199)

That leaves me puzzled - when will the transfer actually end?

Edit: Okay, at some point it ended. Still, what is causing that behavior?

Answer 1

rsync starts transferring the files before it finishes crawling to see exactly how many there are, to speed up the overall process. This means the number goes up as it's running.