When we give Edit only permission on custom record, he can edit his own or his subordinates records.
Use case : employee A -> employee B -> employee C
B is supervisor of C and A is supervisor of B
So when we give Edit only permission A can edit records created by B and C along with his own records. B can edit his own and C records.
Now if A tries to access record created by C, we don't know whether he is supervisor to C. We only get to know B as record owner supervisor. (By searching document owner supervisor).
In the same way if there are multi level hierarchy we can't determine whether top one is supervisor or not with simple search.
Core by default will take care of this for custom records. But as we are working on suitelet (on clicking edit or view we are opening suitelet) we have to take care of that functionality