This is not unique to Ball Pythons, I have had a King Snake, a Corn Snake and a Rosy Boa do the exact same thing to me.
Obviously not on the same day, at different times over the years.
When a snake starts that slow exploratory sniffing, I immediately stop them by moving them to another position.
More often than not, I think that the slow innocent sniffing and almost nuzzling, is the pre - cursor to that exploratory bite.
I must say however, that in the case of the Rosy Boa, it went into a full feeding response with constriction of my arm as well.

Just my two cents worth.