Timing Light

OK, I 'm using my Dial Timing light, don't have the brand or model, to set my initial timing. Car seemed to want more so I got it to about 16* btdc on the dial. When I turn the dial back to 0 the light was hitting the mark at 10* btdc. Is my timing light way off or is something else going on. I've checked the marks and they are spot on with TDC. Any thoughts.