What frame rate does the info panel report for the clips?
I haven’t shot with the Canon but I know with my Blackmagic camera, if I shoot off-speed, eg. my frame rate is 25 but I shoot 60, the camera is capturing at 60 fps but writing a 25 fps clip to the SD card, giving me slow motion. As far as FCPX knows, that is a 25 fps clip.
Assuming the C300 is doing the same thing, you may have to look for a metadata tag other than frame rate to see that it was recorded off-speed.
Jeff Kirkland | Video Producer & Cinematographer
Hobart, Tasmania | Twitter: @jeffkirkland