When an user checks out, it seems that there is a minute missing from their registration afterwards.
There can be several reasons for this. The most common is that even though it isn't shown everywhere, checkins are precise down to the second and will round a resulting time registration based on this. So if a user checks in at 08:00:45 and checks out on 08:30:05, there have elapsed precisely 29 minutes and 20 seconds. As hour registrations are only precise to the minute, this will be rounded down to 29 minutes.
This will somewhere in the system show as a checkin from 08:00 to 08:30, but a time registration of 29 minutes.
Another thing to consider, is that time is counted from the exact checkin, so there will typically be a small gap between a checkout and a checkin, as the user needs to locate the next task.