A) Resolution differences
B) Your computer is ancient
C) Are you really encoding something that's typically 3 or 4 minutes long, or an entire 24m episode/2h movie?
Take your pick. The rule of thumb is not to trust any figure someone pops out unless they also give the parameters (video length, video resolution, hardware specs) they were targeting as well. All those things are variable, but in no way will veryslow be faster than the other presets (except placebo, which is the only one slower than veryslow).