Well, I imagine someone told you that b/c technically a horse's canter stride is said to be 12 feet. (personally, I think that's a little long for lots of horses) Technically when you're building a course (of jumps) you set 6 feet for landing after the first and 6 feet for take off for the next. You'd increase for bigger jumps b/c the take off and landing might be further then 6 ft. (I keep 6 ft if it's about 2"9 and under). Caveletti's don't have any spread to them so you space them at 9 feet. Although... now that I think about it I guess it doesn't really make sense if a horse's stride is supposed to be 12 feet... not sure about that one!
Ha, I just know that that's what you're "supposed to do" and that it works!