Git doesn't set special options to it's pager command if the LESS environment variable is set. I noticed when I started using Gentoo that git was putting all output into a pager even if it were less than one page, which it didn't do under Ubuntu. A fellow Gentoo user confirms this; apparently Gentoo's defaults set LESS, but without a -F.
Should git always tack on a -F when it invokes less, or should this be put on the user to look up what to do? Or should I go complain to the Gentoo folks?