depending on how close you hold it will affect the reading too. the closer you hold it the finer the surface area it reads from if i hold it 1/2 inch from the substrate compared to an inch i can usualy see 1-2*f difference as well. 15*F might be a little bit extreme but i could be wrong could be 100% normal. i dont get that huge of a difference. but its at lease 8*f difference for me