Apparent brightness vs luminosity
Deneb is an ideal lesson in the difference between how bright a star looks and how much energy it actually emits.
It appears bright from Earth despite great distance, which implies enormous intrinsic luminosity.
Why estimates vary
Distance is the lever. When a star is far away, small uncertainty in distance can produce large uncertainty in luminosity and radius.
That is why Deneb's published properties are often shown as ranges.
Why it belongs here
Deneb demonstrates that supergiants are not only the huge red examples. High luminosity and spectral class are the key ideas.