Actually it's because you live in a city where the transportation department cares about bike traffic. Loop detectors, even the single-loop older style, can almost always be adjusted to have enough sensitivity to detect bicycles (at least ones with either aluminum or steel rims). But if they aren't careful with the adjustment it might be too sensitive and also pick up vehicles in the next lane over - so you might trigger the green arrow for a left turn even if that lane is empty. Many cities don't bother with the careful adjustment needed to trigger for bicycles but still avoid false triggers, or they might set them correctly initially but neglect to do the periodic maintenance to keep them in adjustment. The multi-loop style makes the job easier since they are much more sensitive to metal directly over them as opposed to metal over to the side in the adjacent lane.