On a Moodle instance with 3.4.2+ (Build: 20180425).
I added a positive offset to a Quiz grade item and to my surprise some students got more than 100%, which I had understood was impossible in principle in Moodle.
I tried to replicate this behaviour on the Mount Orange Demo site and on another 3.7 site but without success. In these cases, 100% IS the maximum.
I cannot find any reference to this bug (if it is a bug) on the Moodle Tracker. As it only affects an old version of Moodle, perhaps it is not important anymore.