Grendel insists that his death is an accident because he slipped on the blood of Heorot and so Beowulf was able to take advantage of his misstep. Therefore, it is not that Beowulf is actually stronger than Grendel; instead, Grendel's loss to Beowulf was only a slip on his part.
I just read Beowulf in my British Literature class like 3 months ago :)