You could have you bot upvote, then remove its vote, and monitor from another browser whether the score changes. Having the number be fuzzed makes it harder to confirm if your bot has been caught and it's votes discarded.
Fuzzing the votes like this is cheap for reddit, but makes it more expensive for the spammer.
The best spam-filter is tricked by a hand-crafted, personalized email that is well-targeted. However, the cost of these are insane compared to mailing the same thing to hundrets or thousands of users. So anything any spam-filter ever does is making spamming less lucrative for the spammer, and fuzzing does that.
But you're multiplying it by some number for every single comment/reply. Spammers work through large numbers... make each one a little bit harder, and it adds up.
A votebot suspects that it is being detected and filtered out of the final scores. Because the scores fluctuate, it is harder to determine if your vote had an impact.
But that doesn't "prevent spambots etc.", that merely prevents people from easily and instantly figuring out whether their bots are detected yet. It doesn't stop them from spamming votes, or from making their bots more elaborate regardless of whether they have been detected yet. I don't know all the motivations for spam bots or how people who make them tick, but I'd figure for a significant number the crucial bit is having an impact, not measuring it. Most spam is fire and forget, after all.
The point isn't to prevent them which is near on impossible but to get them to waste enough resources on non-visible actions regularly enough that it isn't economically viable to continue trying to spam the site.
