Altitude. While you will get 2400 RPM out of a C-172 at sea level, you won't get 2400 RPM out of that engine at an airport located at 4000 feet asl because the engine is normally aspirated (not turbo/supercharged) The information is in the C-172 POH if you dig deep enough... therefore it is a range of acceptable power depending on altitude.
Sorry, upon rereading your question, the speed is what causes the rest of that difference in RPM, as stated by +TSRA