The diff depends on the pool, and even the socket. Even then if the dev fee uses the same pool and socket, but it's a variable difficulty socket, then the difficulty can be set lower when it makes the new connection and probably won't even settle in to your normal difficulty by the time the dev fee mining is over.

That's why you should look at the amount of time spent dev fee mining, rather than purely the number of shares. Number of shares doesn't mean anything if you don't know the difficulty of each.