There's your problem. You were too far away.

It reads 3600 when pointed at an incandescent light because that's the only light in the room. The entire room is flashing at 3600 RPM. Outside, only the relatively small propeller disk (as compared to the whole wide world) is flashing.

Is the TNC Tach guy considering selling the design? If they're such great units, somebody should snap up the design and start making them again.