The new delta code needs a 16-byte window to match, so to *know* that there will be a match, you need ~32-bytes in common. (guarantees that 16-bytes somewhere in that 32-byte range will match.) Also, when setting 'max_delta', it is possible that we run out of bytes before we actually find the last match, which would make things compress better. This is rare in practice, because texts are longer than 40 bytes. But it happens in testing.