Why would the number of files in a backup not match the line count of the Labels.txt file?

I'm not sure this is a bug, but I've noticed that the line count in Labels.txt does not correspond 1-to-1 with the number of emails downloaded.

Can someone shed some light on why this would be?

Its off to the tune of (in one case) 33,000 files and 17,000 lines in the labels file.

Thoughts?