How deep do you need to insert the probe in a piece of meat to guarantee accuracy?
My theory is that the probe should be inserted into the part of the meat that will be the last to heat up. For a roast without bones, that will be pretty much be the center of mass.

For something with bone it, I would aim for a point somewhere between halfway and 2/3 the distance from the skin to the bone in the place where that distance is the largest.

Are you checking the temperature while the meat is still in the oven/on the grill? For a large meat where the entire probe is in, there will be no effect. But for a thin piece of meat with just the tip inside the meat, the air temp and radiated energy will throw off the results. Either insert it length-wise in between the ribs, or set the ribs on the counter to check the temp.

If you are still getting inaccurate results after this, then I would suspect the accuracy of the thermometer.