Usually rc2 is the really quiet one, but this release cycle rc2 was fairly busy and it made me worry a bit about whether there was something bad going on with 4.13.
But no, it was just random timing, and people got started sending in fixes early, and this release cycle it's rc3 that is small. It's about half the size (in commits) of rc2 - usually things are the other way around. Maybe people are starting to go on vacation (August tends to be quiet in Europe in particular).