Union code bug or feature?

Summary

How union code is used in calculating earnings/benefits/etc

Content

Not sure if this is a feature or a bug. It's not causing us any grief at this point but thought I'd share my findings with the community. 

We use the union code as a way to define what amount of bonus(earnings) an employee will receive. Over the course of a year that could change. What I noticed when testing and then validated by reviewing the calc rules. Is that The union code is looked up based on year total. What this means is that that the last union code on the employee record will be used for calculating the earnings or other benefits that are tied to that union code.