Downvotes used to be public on reddit, too. They changed that, and further "fuzzied" the visible total vote count, because reporting those numbers perfectly accurately and publicly made it easier for astroturfers and spammers to game the system. They were able to see the effects of their sneaky efforts, which helped them identify what worked and what didn't, which then allowed them to know which sockpuppet accounts were shadowbanned in some way.