The sequence, at least in theory, has to do with the relative size of page elements. If you accept 13 units as the size of a headline and work backwards in the sequence, a Heading 2 would be 8 unites, and then perhaps text would be 5.

But the relationship between math, nature, and a civilized person's perception aren't always in synch. IOW, nice theory, but it doesn't always play.

-g-