In a still air, it is very important where you take the temperature. Hot air rises. In an incubator with a fan it gets mixed up so you can take the temperature anywhere, but in a still air, you need to take the temperature right at the top of the eggs. The recommended temperature there is 101.5 instead of the regular 99 in a forced air.
If you have not calibrated your thermometer or if you were taking the temperature lower than the top of the eggs, it is quite possible your incubator is running a little warm. You can either make sure you are using a calibrated thermometer (medical ones are calibrated) and take the temperature where you re supposed to, or you can measure it exactly as you are and tweak the temperature down a half a degree to a full degree and see how your next incubation goes.
Each incubator is different, even the same make and model. You can tweak an incubator to where it is working perfectly then just move it to the other side of the same room and get different results. You can use science to get to a good starting point that will work pretty good but there is a bit of art in the final adjustments. he good thing is that you don't have to be absolutely perfect to get pretty good results.