This has happened to me several times.
Alas, there is no way to see who exactly the upvotes are from, so there is no way to tell for sure.
Your question has 1475 views, as of this writing. More views $=$ more upvotes, period, even on a mediocre question.
Your question is about a well-known topic, and people upvote things they understand. It is a well-known phenomenon on mathSE that more advanced questions and answers get less attention simply because less people follow the topic of those questions.
People don't necessarily refrain from upvoting just because the question is a duplicate.
Upvotes don't always come from established mathematicians or mathSE users. In fact many users from other SE sites come and generate a lot of uninformed votes (exhibit A).
With regards to the first bullet point, where did all those views come from? Well, probably from the users from other SE sites, see bullet 4. In particular, what often happens is:
The "Hot Network Questions" list attracts views from users outside of mathSE, from the SE network in general.
The association bonus allows these users to vote, even if they are completely uninformed about math or about the customs on mathSE.
This has become a significant problem on MathOverflow (1, 2) as well, but the proposed solution here (which I have upvoted) has mixed feedback.
The problem is that votes are simultaneously serving two conflicting roles. On the one hand, votes are supposed to indicate how good or valid a question or answer is, in which case we only want established users to cast them. On the other hand, votes indicate that an answer is helpful or that it solved a person's problem, in which case we want any average user (i.e. a user trusted on some site in the SE network) to be able to express that an answer was helpful.
In general, I would not expect the vote total on any of your answers to accurately reflect whether the answer is good or not, except in the case of low vote totals like +1, +2, -1, -2.
This proposal for example tries to address this.
See also: If you worry too much about voting patterns on this site, you will lose your sleep.