There is always a learning curve for reading english even though it is my primary language. I missed this one. Check it out and comment. GW mistake or done on purpose?
"One model per three reavers may take:"
Reaver beeing the name of the unit model not the jet bike they are riding (thats a "reaver jetbike") and the upgraded "champion" is named Arena Champion and thus is technicly NOT a reaver and does not count. My guess is that this would have been overlooked if it wasn't for the other option about the shooting weapons where the text says "For every three models..." and not every three reavers.
So what this means is that for every 3 models 3, 6, or 9 you can upgrade a weapon. If you upgrade a reaver to an arena champion you need 3 reavers + the champion to upgrade a single cluster caltrop. With the upgraded champion you can only get your caltrops at 4, 7, and 10 models.
What do you think, incoming errata or clear as day?