The composite rankings are not averages of their three different rankings, because you would get a lot of kids “tied” for the same ranking and wide gaps in the numbers themselves (10-13 could be vacant, for instance).

The easiest explanation in this particular case is very wide variance in the Rivals and Scout rankings, i.e. there are recruits in either of their top 50’s that did not even scratch the top 200 on the other sites.