Yes as the anti-rootkit does check a lot of files, though it is surprisingly quick at it, mine is normally very quick, 27 seconds for shed load of files scanned.
My on-demand Full System scan only does the Rootkits (quick scan) so I guess if a) your system has more data, b) or you are doing a custom scan where you elect for a Rootkits (full scan). Either of these may mean more data is being scanned.