The EYE-TA offers two types of scores for each child: the EYE-RTI (Responsive Tiered Instruction) score which is age normed and the individual domain scores for each of the five subdomains of the assessment which are not age normed. The main reason for this difference has to do with the grade structure schools adhere to.
The assessment itself is skill based and the domain scores produced are intended to measure a set of skills that all children will need when moving into Grade 1. As graduation to Grade 1 is the same for all children in a given cohort, their specific ages at the time of assessment are not the important factor here. It only really matters that they can master the skills required by the end of the school year.
The RTI, on the other hand, uses these domain scores along with a child's age at the time of assessment to produce a single score that is an estimate of the probability that the child will become a successful reader by the end of Grade 3. It is age normed primarily to account for differences in physical maturity which can play a large role in a child's ability to complete some of the assessment items. Younger children are expected to have slightly lower scores overall, but that does not necessarily mean that they are at increased risk of not learning how to read on time. It also allows for meaningful comparisons if data is collected at multiple points during a school year (typically once near the beginning of the year and again near the end). Since the children might be half a year older at the second data point, it makes sense to adjust the scoring to account for their growth during this time.
For more information on the domain and EYE-RTI scores, please check out the following article: Understanding the differences between the EYE-TA domain and RTI results