The maximum of 'something' is infinity...if you think about it. 100% just means "everything" or "total"....110% means "a bit more than everything" or "more than the total".
Engines can run at more than 100% because they are very powerful. Engines are however seldom run above 100% except when an extra bit of "umf" is required, say in takeoff (tho the engines are derated most of the time), go around, recover from stall etc.