Just to clarify: Grainfields have a higher TU cost to traverse than grass (6 TU vs 4 TU). Thus, given the opportunity to skirt it, the second path walks on the grass beside it, then around the wall through one tile of grain. In this situation, you might have troops in Flying Armor go up one level and fly over the grainfield and land on the otherside as their preferred path, as well.