I see blue tile on the wall, penny tile on the sloped edges that transition to the base of the pool, hexagon tiles on the bottom of the pool, and the brown edge is what happens when a drained pool collects rain water and dirt. Drained pools usually have some sort of nasty dirt stain or mosquito cesspool in the deep end.
My guess is that they picked the bottom and side tile, and didn’t realize that large tiles are a pain in the ass on curved surfaces. They require a lot of time to cut and mortar. Sheets of penny tiles are already cut into something that can flex on a curve.
But sane people usually continue the penny tile up one of the flat sides so you don’t have a hodgepodge of different stuff.