I’ve tried using my thermometer while cooking Italian Porchetta and it will register that the meat is done, but actually the meat isn’t cooked, why?

If the tip of the thermometer probe is too near the surface of the meat cut or near a bone, you will not get an accurate reading. Experiment with the temp probe placement – it is most accurate when the tip is in the center of the meat portion. If you have several cuts of meat, move the thermometer to several portions to compare how even everything is cooking.