@Riot: Any reason Garen's Spin doesn't scale past lvl 16 in URF mode?

The ability description says that the ability gains an extra tick of damage every 3 levels, so it's somewhat odd to me that Garen can't go above 10 ticks of damage despite being able to reach lvl 30 in URF mode. This is made particularly perplexing by the fact that Gangplanks Barrel tick rate indeed goes down beyond lvl 18, namely at lvls 19 and 25.