Only ~90% of my session have user ID.
I am looking at my user ID view with sessions as my metric and user ID (a custom dimension) as my dimension. I noticed that that the sessions only represent 91.06% of the total sessions. I don't understand how this can be the case since I am on my user ID view where every session must have a user ID. I'm worried that somehow user IDs aren't getting reported for some of my users who are logged in.