No impression number can generally be taken to indicate the first printing. Same as a "1" or a full number-line. That said, I doubt it was a conscious decision, and more likely it just got forgotten.
Given they list Finn & Hengest signature as a 2006 Edition and it has the same ISBN as the 2006, it would be annoying if they truly considered that the 1st impression.
HarperCollins has been quite inconsistent with how they treat the signature paperbacks. Sometimes they've done a new ISBN, sometimes the same ISBN but a new 2024/2025 edition (sometimes but not always resetting the impression number) but sometimes they've actually reverted to an earlier edition. For example Lost Road signature is a 37th impression of the "1993" even though there was a 2002 and 2015 and the impression numbers continue from the 2015!
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