Version name issue

I currently have gimp-app gimp-app @2.4.0-rc1_0 installed. If I do a sync and then port info gimp-app it shows gimp-app 2.4.0. But if I do port outdated, this port does not show up as outdated. I would think that 2.4.0-rc1_0 is older then 2.4.0. But port seems to think that it is more current. Is it just doing a string compare to find the which one is newer? Am I correct about which one is in fact newer? Thanks, --Adam