The position of the tiles in the brush tileset is not indicative of what the auto tiling will do. That's just the place each tile needs to have so the algorithm works.
The tiles are chosen such that the outer region of the drawn tiles always have the "grass" part and any inner part has "water".
Notice that whenever a tile is placed on the tilemap, all edges of the tile which have an adjacent space which is an empty tile, end up being some form of "grass" while any inner space is "water".
Ej 1. A single tile with 8 empty tiles around it will always be tile "8". All "grass" in the edges and "water" in the middle.
Ej 2. A single tile with 8 drawn tiles around it will always be tile "64". All "water" no "grass".