Usually, these are references either to statistical distance from the mean, or level of functional impairment. For example, he might have meant that inattention (and other ADHD-related symptoms) was (and I'm pulling these numbers out of a hat here) at a level seen in only 2% of the population, but reading difficulty was at a level seen in 10% of the population.
If functional impairment, then he might have meant that your DC is able to limp by more or less with reading comprehension, despite actual word-level reading difficulties, but that inattention is noticeably interfering with his access to instruction, or skill acquisition, or peer relationships.
I agree that remediation/accommodation for both is important, but I would also consider the value of working on the most severe challenge first, as it's probably the one soaking up the most of your child's energy, and affecting his educational experience the most.
