Lostinlondon, don't worry, I think it is correct that they have a fair idea. So far with boys that were awarded sports scholarship at 10+ last year, from what I've seen in the term and a half so far they have lived up to expectations. There are a couple of boys that are really good not on a sports scholarship. But I can't say anyone that has one, so far anyway, hasn't deserved it.
I have to say though, I have heard this year ( particularly in11+ group), the competition is very steep, and most talented boys sports wise want whitgift. I heard also in both 11+ and 10+ group, there has been an unprecedented number of sports ( and music), scholarship applications. A number of boys I know who are very good ( county level, cricket and academy footballers), didn't get callbacks, so your DS has done really well.
I also believe they are looking for boys who are well rounded, and not just good at 1 sport.