Outside user set to view certain leads based on a field selection. Why are the numbers of those that he can see less than what we see for the same field selection? BPM criteria incorrect?

We have an outside person we work with that is set to view ONLY a specific type of lead based on a field selection yet there is a difference in the number of those leads that we see vs. what he sees and it's the same field selection? Could it be an issue with the BPM criteria?