Currently, the comment for which I've received the most positive karma by a factor of four is a joke about institutionalized ass-rape. A secondhand joke, effectively a quote with no source cited. Furthermore, the comment had, at best, tangential relevance to the subject of discussion. If anyone were to provide a detailed explanation of why they voted as they did, I predict that I would be appreciative.
Based on this evidence, which priors need to be adjusted? Discuss.
The individual upvotes are not independent of each other. A post can be thought of as having an "upvotability" related to its quality, snappiness, originality, conformity to group ideas, adherence to conventions etc that can be expressed as the probability that a random (registered) viewer will upvote it. The total upvotes of a post are determined by the number of viewers, the upvotability and chance (people are also influenced by the current karma of the post and their opinion of the poster, but I think we mostly agree that one ought to try to compensate for that).
Calling the fact that a post received a lot of upvotes "poisson noise" implies that you consider random chance a better explanation than a high upvotability and that your posterior for the upvotability of the post is (close to) unchanged (at the very least it implies that your posterior is closer to your prior than to an estimate based on (net- ) upvotes and estimated number of viewers [and an estimate of downvotes]). Does this match your beliefs? Otherwise guessing the password sounds like an accurate description of your behavior.
EDIT: You might also think of upvotability itself as poisson distributed, but that's a non-explanation equivalent to answering a question about what geological processes cause the Himalaya to be the highest mountain range with "one mountain range has to be highest and that just happens to be the Himalaya".