2000 Altima won't pass timing test

I have to get my car registered in CA and the timing has to be at or near factory specs, 20 degrees. When i took it to the smog test only station, they said it was reading 10 deg. So i went to another shop to adjust the timting, but they told me it's fine, 18 deg. and within specs, so i RETURN to the test facility and its reading 10 deg. again. So the repair shop sends a guy over with his own timing light, and sure enough now it's reading 10 deg. again. What is going on?