The tendency of heroes facing a powerful enemy belonging to some organization then after villain is barely defeated, he claims that he was only weakest member in the group. Purpose seems to be to make villain group seem powerful by having even their weakest members be strong.
Sometimes its not the member itself who say they were the weakest, but the stronger members. Also, member doesn't need to be always defeated, its possible for him to mock the heroes after defeating them with this.
Kinda similar to Sorting Algorithmof Evil
I guess, though this is of the cliche for them to state it. Also similar to Those Were Only Their Scouts