If an account is upvoted because it’s posting high-quality content, we’d expect those votes to come from a variety of accounts that don’t otherwise have a tendency to vote for the same things.

Suppose you do regression analysis on voting patterns to identify the unknown parameters determining how accounts vote. These will mostly correlate with things like interests, political views, geography, etc.—and with bot groups—but the biggest parameter affecting votes will presumably correlate with a consensus view of the general quality of the content.

But accounts won’t get penalized if their votes can be predicted by this parameter: precisely because it’s the most common parameter, it can be ignored when identifying voting blocs.