What unit are you using to measure temperature? Looking at an image of this incubator it looks as though it has a small vent hole on top. Put an oral thermometer down that hole to get a true reading. The temp swing is heater turning on then off. Try to get the swing range with the oral thermometer. That's the temp you can trust. Once you know how far off the unit is reading it will be constantly that far off. Simple addition or subtraction from reading for true temp.
The incubating temperature is the average of those temp swings. Your unit is telling you 97.8-99.4. That's an incubating temp of 98.6F which is a degree low. Calibrate temp, ensure what it actually is with the oral thermometer and adjust unit until average is 99.5 F because the unit has a fan that's the temp throughout incubator.