First readers as opposed to early readers: what does it mean?

I figured it would be useful to explain why I'm putting first readers back with early readers and what all these distinctions mean.

The head of Children's prior to Eileen took the very early readers-- e.g. The Fat Cat Sat on a Mat-- and labeled them "first readers" so parents who had beginner readers could find the books more easily instead of ruffling through the entire early readers section. However, there was never clear criteria for why a book went in one collection as opposed to another. There were some early reader series that had books in both spots. For example, there is a series of early readers called "Rookie Readers" and there would be some in the first readers collection and some in the early readers collection. It was all very confusing. When Eileen took over as head of Children's, she didn't like the inconsistency and decided to merge the two collections. Indeed, both collections are for beginning readers, so why have them in two locations? That is why I'm putting them all together again. The collection will be much more cohesive once this project is finished.