Good point Average_J, the terms get confused frequently.
Keith S, my experience has been that at least 9 course layouts in 10 that I certify were short. Seems I'm always adding distance.
I just tried calibrating a new bike computer on my mtn bike for a course I'm measuring (but not certifying) and found it measured short. Before I actually measure, I'll check it again to see what number I'll use for the mile marks. Right now it looks like 1.03 per mile. Point being, I take the time to calibrate a mechanism that'll help me get a mighty accurate measurement (on a course not conducive to certify currently).