The way the pathfinding works is that the game has a list of tiles to check ordered by how long the walk to them is and how close they are to the goal, then checks the 4 tiles around each of those tiles, and if they're not inside walls or such they're added to the list as well. There's a priority order in which direction around a tile is checked first, and on a map as small as this, multiple tiles are probably likely to get the same "distance from goal + distance to walk here" score. These factors can create situations where the game can favour a path that to human eye looks unoptimal.