How are POINTS awarded?

[BuzzBuzz]

I have been trying to figure out how points are awarded for solving cryptograms. So far I have solved 44 puzzles, and I have plotted a graph of the points awarded vs. seconds to solve. My main conclusion from examining this graph is that there are several categories for difficulty. I am not quite sure how many, but I think there are at least five. For each category, the points awarded is based on the following formula:

points awarded = A – S * B,

where:

A is a point value constant that depends on the difficulty category,

B is a points per point constant (which is about 1/6), and which may or may not depend on the category,

and S is the number of seconds taken to solve.

What remains a complete mystery to me is how a category is assigned to an individual puzzle. Can anyone please help me figure this out?