Possibly. Keep in mind, though, that being able to perform at the 90-95th %ile in math does not necessarily mean mastery of skills qualitatively far above grade level, especially in the early grades, where much of the skill difference from grade to grade is in degree only, not in concept (e.g., if you understand single-digit addition and subtraction, there is no particular conceptual reason you would not be able to grasp multi-digit addition and subtraction, with and without regrouping--yet the average age-peer student does not achieve skill mastery of the latter skills without direct instruction and extensive practice). So a child who truly understands the basic math concepts of first grade will not all that startlingly be able to apply and extend them to skills not taught until one or two grades later.
Also, the average student does not show independent mastery of grade-level skills at their instructional level. They show mastery of skills somewhat below instruction, and developing skills for those for which they are currently receiving instruction. The norms order the examinee against the general population, not against grade-level instruction.
So yes, it's a bit natural ability, but it's also that your DC probably actually understands simple addition and subtraction, where most age-peers (certainly the ones in the US and Canadian WIAT-III norm groups; you may be able to speak more to the norm group wherever you are) are just implementing taught procedures. A symptom of one of the pervasive flaws in mathematics instruction in many western educational systems.
