Hi,
If user b has user level right on account but still can see accounts owned by other users then user b must be inheriting the permissions from somewhere.
Most probably the records are shared. You can isolate this by creating a new record and see if that also is visible.
Other possibility is that user b is assigned to a team which has read access to those records.
In short, you will have to figure this out by hit and trial method. Give the same role to other users and see if it is the same behaviour; check if this is only for few records. remove 7 assign role agan etc
Hope this helps.