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.
