Maybe there's a lot of bidders. You rather check this https://steembottracker.com/
So you can able to check all the bots bids that have a small bid.

Sometimes the bot only can vote for 10 users for example, but the bidders are 25 users, that's why sometimes the upvote of the bots will be divided to the 25 bidders.

The High your bid the more chances of getting higher votes from the bots.
But today bidding on the bots is not safe because the exchange of the SBD is falling down that's why the votes of the bots sometimes got affected.

maybe on the next month/s this will be resolved.