Does each upvote (by a user with more than 125 reputation) really raise the score of an answer or question by one? Or does one "unit" of displayed score really represent a much larger number of upvotes? (I don't understand why the vote score of some questions is so low.)
Voting up requires 15 rep, not 125.
If a post has say 6 next to it, it means it has a net votes of 6, that can be +6, or +7/-1, for example.
>999 rep users can actually view this breakdown.
A single up/down vote from any user that can cast a vote is just +/- 1 on the post, and +10/-2 rep points to the author of the post. And users can't vote more than once on a post either.