look at the snapshot
I can guarantee all the fonts are of same style, same size, same font. But OneNote tends to render it different sometimes the notebook is opened. It applied to all Pages in my notebook. Very weird!
my default font is Calibra 11. The symptom may not be very noticeable from the snapshot, however it is very obvious in the screen. And u can open the pic in new tab to look at the screenshot in original size with clear sight.