You could be slipping slightly. You can compensate for a little yaw with aileron or vice versa. The yaw could be due to rt thrust or a slight asymetrical control setup. Make sure its also not a cross wind causing it. Even if there is little or no wind on the ground there can be more wind and from a slightly different direction at altitude. Get with one of the more experienced flyers and see what they think. If you don't fly with a club then check to see if the crab is always the same direction in relation to the airplane and not the ground. If it is for sure the plane try removing crab with the rudder trim and keep it level with the aileron trim. With no wind or directly into the wind it should enter and exit a loop from straight and level on the same line. If the trim is still off it will do more of a large barrel roll.