I think there is one legitimate reason to vote something a zero that isn't trolling: if you are voting with the final result in mind and you know math.
You can either vote exactly what you are thinking because you want your voice to be heard and damn the results, or vote with some thought based on how you can affect the final score to best reflect what you feel.
Same thing happens when we vote for GotY on the staff forums, some people just vote with what they think should win regardless of its likelihood to do so (me) and others hang back, count points/votes and try to affect the final result to come as close to what they'd want it to be as possible (Machina). Neither of these is particularly wrong, and
I view it the same for voting on a user score. If all you care about is that the user score gets as close to the number you think the game deserves as possible and you know how averages work I could see someone voting 0 for a game that obviously isn't a zero, but not doing so in an attempt to troll.
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