[jewel]

The disk cache is what lets the second one catch up. It would only happen if the copies were started at nearly the same time; soon enough that what's been copied so far is still in the cache.

I've observed this same behavior on modern operating systems with big rsync jobs and spinning drives. If you have lots of small files then reading the metadata and directory structure takes a while but fits in the cache, so a second rsync will catch up very quickly to the first.