The short height of the plug is what lends itself to rounding, especially if using a 6 sided socket, or the often "close enough" standard size when not having a metric set on hand.
Well, "what lends itself to rounding" is the idiot that over-tightened the plug!! I tried a 6-point impact socket, and the drain plug on my W56 transmission would NOT move.
Gnarls.
