mongodb/mongo-csharp-driver — comment on commit 4c1b6d2a80cf1b9b87157b9d758ebe63d864b9ef
File:  line 0
By craiggwilson:

@robert: I pass even the generic versions down to the non-generic method. But, I did put some breakpoints in to see when we didn't come from the generic versions and most of them are like this.

The pattern seems to be any time an IEnumerable is returned (generic or otherwise), it goes through the non-generic version. Count seems to go through the generic version. Ultimately, the take is away is that you need to make both equal in all regards.