Page 107 - Turkey Book from Meathead

Grill manufacturers compete to undercut each other's prices, so they are not about to install a high-end thermometer, especially when most of their grills are used for hot dogs and burgers. They buy bargain basement. Even the top brands use cheapo thermometers. Look at the picture above. Some of those are very good cookers. But their thermometers are not.
Dial grill thermometers are not only inaccurate, they are also mounted in the lid, way above the cooking surface. Well, the temp in the dome can be a lot dierent than the temp at the grate. The grate is much closer to the heat source and the dome is closer to the ambient air. If you are cooking on Thanksgiving Day north of the Mason-Dixon Line, that air temp is cold.
And from left to right, the probe is in the middle, so if you are using two zones, it is giving you an average. Not very helpful.
Either indoors or outdoors, we use two thermometers when we cook, a handheld instant read thermometer and a remote read thermometer with more than one probe. The key to turkey success is to avoid overcooking or undercooking it even the slightest bit. And yes, you can poke it many times and it will not dry out. It is 75% water, so a 20 pounder is 15 pounds of water. A few drops escaping through thermometer pokes will not make your bird dry. Proper temp is far more important.
AmazingRibs.com has an electrical engineer tricked out with special lab equipment that precisely measures the speed and accuracy of thermometers. He is also an accomplished cook.