If there's draw the game actually adds 0.1% to a random team. (Although usually the entire stage doesn't get inked to there won't be a total of 100.1%)
In other words, if you lose or win by 0.1% then it might have actually been a draw.

In his defense, it's likely exceedingly rare for a normal (not intentionally idle) game to come up in a perfect draw and the trouble it would be to code in a tie breaker, make extra animations, throw in new sounds, etc. etc. would be much more trouble than it's worth.

I don't know if Splatoon has added new game modes that are more likely to end in stalemates, but for an average turf war, I think the current solution is probably the lesser of two evils. I'd wager it's more than just totally random if there are any inconsistencies with stats between the two teams as well.