This is a result of the binning process. The 7-core GPU versions are TECHNICALLY defects. There are almost certainly also 6 core, 5 core, etc. But salvaging the 7-core "rejects" allows Apple to recoup some costs, and pass some savings onto the consumer.

There's a great explainer about chip binning in general here: https://www.techspot.com/article/2039-chip-binning/