I am using the API to return session data for users (identified with a custom dimension).

I have used the GA Query Explorer to simulate my query.

When I do so I get a different result from that which I see in the GA interface. Specifically, many user ID's return 0 sessions even though the GA interfaces shows that user with activity and correctly returns the session. In other cases the number of sessions returned via the API is lower than that via the interface.

Please note that the Query Explorer confirms that there is no sampling in place on the data I am retrieving. As you can see in the screenshot below some users are showing 0 sessions - yet when I check that user in GA I get a session total and I can see the associated pageviews