Map selection in Splatoon's online multiplayer is randomized in a peculiar way. Instead of being randomized between all the game's maps, only two are available at a time, and the computer will choose between those alone. These available maps are cycled regularly. It's a curious method, but according to the game's creators, it has purpose.
Director Yusuke Amano explains:
"I think if maps were selected at random from all the possibilities, everyone would only use weapons that have “the greatest common divisor” and that work the same way in every map. But when maps are limited to two, you can have alternative weapon choices like “I’ll go with a charger for these two maps”. If there are more than three stages, all-rounder weapons become popular." — Yusuke Amano