This means it is receiving a hash that it didn’t need. The question would be more why this hash wasn’t stored to the database and then re-used…
W.r.t. the original question. The parallel hashing concerns (from the manual):
If checked the client calculates hashes for each file before the backups (only hashes of changed files are calculated). The file then does not have to be transferred if another client already transferred the same file
Without parallel hashing this is done during indexing. With it is done during backup (in parallel), with one thread. The options are to increase the number of threads, but I wouldn’t touch them currently in production.