It's not exactly a bug. Many aircraft that didn't have in-flight adjustable trim for all control surfaces were set up this way, either by a built-in trim from the factory or by ground adjustable trims that were set by the ground crews (this is what the 109 rudder was like). One of the Italian birds, i think the MC.202, even had different wing spans to account for this (one of the wings was slightly shorter and this created a rolling tendency that balanced the aircraft out).

The reasoning is that since the plane cannot be trimmed by the pilot in that axis, it should at least be pre-trimmed to make it less tiresome to fly. And this means that they were trimmed for their nominal cruise airspeed, because it was at that speed that the majority of a flight was conducted. So, flying at that speed range requires no pilot input for ailerons, but flying above or below it needs adjustments.

The reason you don't see the surfaces moving on the ground when you are standing still is that trim works aerodynamically: the trim tab needs airflow over the wing to deflect the control surface.

For example, an elevator trim tab is like a small reversed elevator. If you want to dial in some nose up trim, the trim tab moves down. This pushes the rest of the elevator up and makes the nose rise. If the aircraft is standing on the runway however, the trim tab doesn't work much (apart from whatever airflow the propwash generates).