They are very accurate, but when pointed at something shiny and metal they give the temp of whatever is reflected, just like a mirror. When pointing at a foil covered stone they will report on the temp of the oven roof. The stone is probably way cooler. But that's no problem. That's what you want. When you take the foil off using tongs, then repoint it at the stone, it will probably be 150F lower. more or less. That's great, because that differential is what you need.
Bottom line is that you have to get used to this process. Your oven may vary from mine.
But overall the IR thermometers are very accurate. They are very cool gadgets. Kids will get endless pleasure out of pointing at each other's butts :-) They work on color so distance is no barrier. If you point them at the sky they will go off the charts low (space is cold). You can take the temp of the moon. Of a building a mile away, whatever. They work on what's called the 'black body effect' which says that something's peak radiant color is determined by it's temperature. So white light tells us the temp of the surface of the sun. Heat metal to the same temp and it's the same color. A regular lightbulb filament is super hot, (like 2500F) which is why it glows white.