It is quite common that the CookPerfect ambient sensor shows lower ambient temperature than what your oven or grill is displaying. There are a few key reasons for this:
When cooking meat, the surface area around the meat will have a lower temperature than the defined cooking (air) temperature inside your oven or grill.
This is caused by the colder temperature of the meat cooling down the air around the meat.
CookPerfect is measuring the actual heat your food is subject to and not what your oven or grill is showing or set to.
Locally inside oven or grill the air temperature can vary a lot, so a small change in placement of the Cookperfect sensor can cause that readings of the air temperature will change significantly.
In majority of grills the sensor is place in the lid for measuring the cooking temperature. Here the temperature can be much higher than the temperature surround the cold meat. This is due to the fact the hot air rises as cold air be lowered inside the grill. Therefore specially in a grill you can observe high temperature reading in you lid of the grill compared to ambient temperature shown in you App.