Sure, the sensors collecting the altitude data might have been faulty, but the helicopter pilot was reportedly 50% higher than was allowed. You would think that an experienced helicopter pilot would be able to gauge that they were a full 50% above the allowed limit. From the reported information, I'm not sure there is reason to believe that it was much more complicated than altitude misjudgement on the part of the helicopter pilot, unless they were somehow unaware of the allowed limits in that airspace.