Unless there is evidence of cheating, we really can't assume that it happened.
As a recovering ChessNut, I have beaten players that were much better than I was. Whether that was due to my playing my best or my opponent playing their worst, it does happen. Not very often, but not impossible.
I have had my patzer butt kicked by players less experienced/skilled than I was.
. The later happening more often than the former.
Also at tournament levels, the physical and mental condition of the players has some influence.
Bottom line, there are several reasons why one particular person wins and another one loses with out assuming any cheating.