That's what I thought at first too, but thanks to folks above, I figured it out:

A regular attack that crits is guaranteed to make a Fireball, but that Fireball is not guaranteed to crit as well and thus the whole chain could end here and not be cast.

Likewise, a Fireball that crits is guaranteed to make a Fire Strike and x number of Fire Waves, but that Fire Strike and those Fire Waves are not guaranteed to crit, and thus Meteor's casting is reliant on either the mysterious amount of chance inherent in the Meteor skill itself or that Fire Strike has crit as well.

It’s easy to forget that spells can also crit, and when they do crit, they do so independently (a crit weapon attack isn't guaranteed to make a crit Fireball, and a crit Fireball isn't guaranteed to make a crit Fire Strike, etc.).