Why do my google analytics unique-user counts not match the data?

We've implemented Google Analytics in our site, tracking user-id in both the tracker's userId property, and a custom dimension. The problem is, while the custom dimension shows the number of distinct values we expect, the user-id is being tracked inconsistently.

For example: in a given period where we expect four unique visitors to be reported, we get that from our custom dimension, but the user-id shows only three. Drilling down gives us this report:

There are clearly four rows - as expected - but the total is showing as three. What's going on here? Am I misunderstanding the way GA tracks users?