Discrepancy with favorite counts


According to the API, I have 2,685 favourites.

If I page through the API, requesting 200 tweets at a time, using max_id, I end up with 2,642 statuses.

I’m curious to know the reason for this discrepancy. Is it because a user has deleted a status which I have favourited? OR their account has gone? Or am I just using the API incorrectly