Looks like they choose the levels to make the compression ratios roughly equal.

> For the Silesia corpus, data compression ratios are:

    QAT-ZSTD level 9: 2.76
    zstd level 4: 2.74
    zstd level 5: 2.77

This presumably also means the best possible compression that QAT can achieve is worse than what vanilla zstd can do.