Just because the carjacker story was more exciting doesn't make it less believable. Not by a long shot. Again it's all in how many assumptions you need to make for the story to make sense-- the headbutting story requires much less assumptions about people that aren't Pun-- in fact Pun's the only one acting in a way you wouldn't really expect there. (and we've already established he is a quirky character anyway) Even if you assume that people who are both willing to sing the song and know the song are fairly common to see in a Gamestop (which I wouldn't agree with in the first place, frankly), having a handful of them in the store sing with coordination with no people that don't join in gets to be really difficult. Even if it feels more mundane just the amount of assumptions you need to make makes it less feasible.
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No problem!
This is a cute and pop genocide of love!