DeletedUser76 The way hash functions work is that an accidental change to the input will typically result in the hash output being completely different, but a deliberate malicious change could change as little as one digit.
In the case of a signing key hash, an accidental change is very unlikely -- it would need to be an extremely unlucky error in the flash storage to nail part of the signing key and nothing else. So a complete change of the boot-time display hash is unlikely.
A one-digit change is unlikely too! It would be the result of a very sophisticated attacker. A less-resourced attacker would in general have more hash digits wrong.
Note that the displayed hash is not a hash of the system image, just the signing key. If the system image is corrupted (probably accidentally, in practice, e.g., a storage error or maybe a crash during an OS upgrade) there will be a red "corrupt" message.