The main thing is the massive difference. If the popularity is super close and the battles aren't, say 70 to 30, then either could win (if they had 70)
Thing is though, popularity rates usually have a higher difference between them than win rates. Just use the google trends website (
https://www.google.com/trends/explore#q=/m/0bt9lr, /m/01yrx&cmpt=q&tz=Etc/GMT+3) and you will see that no matter what two items of the same subject you choose, the popularity rate between them will be highly different. In fact, dogs vs cats was the splatfest that had the smallest difference in popularity, and that still ended up meaning that cats had to win 57% of the matches, in other splatfests the things were much more worse.
In the meanwhile, the higher the amount of matches, the higher is the amount of wins you need to have a decent win rate. if you play 10 matches, you need to win 6 of them to get a 60% win rate, if you play 100 matches you need to win 60, and if you play 1000 matches, you need to win 600, it gets more difficult to maintain a win rate the more you play, and in a event that around 400k people are playing together for 24 hours, a team winning even a small number as 55% of the matches is an incredibly difficult task.