Hi! Thanks for your kind words :) The FOV measure in this app is... kind of tricky to achieve because is based on subjective data (your eyes). Being said that, those 2º increments are for both eyes. This means that you are moving the red bar 1º per step, each eye, but as you have a stereoscopic view, the total HFov is spreading along the whole, completed view, so that's why the FOV is increased by 2º.
I have another free app (I have to find the exe) to measure it, not by degrees but with real symbols, painting with your hand exactly the limits of your vision, both looking directly and peripherally speaking. I didn't publish it besides reddit some years ago. If you are interested, I can try to find it and post a link here.
Regards!