That doesn't necessarily follow. It could be that the three knight containers are given priority and will always be chosen.
The impression I get is that the Grail does prepare in advance who gets what class, to try to avoid clashes etc. Shirou, for example, gets Saber as the final servant summoned, despite it being the class everyone wants. But, he was actually guarenteed to summon Saber from the moment he got the command spells, if not before, due to Avalon. It seems unlikely to me that that is just co-incidence.