GNU ddrescue "Rescued" is larger than the HDD
lsblk uses legacy power-of-1024 prefixes (K, M, G, T, etc.). GNU ddrescue reports its progress with power-of-1000 SI prefixes by default (k, M, G, T, etc.). 596.2 GiB is just over 640 GB (a common capacity for laptop HDDs). 640 − 633 = about 7 GB not rescued.
lsblk -b (or lsblk --bytes) will report exact sizes in bytes. Alternatively, ddrescue can use powers of 1024 and IEC prefixes with the -B (--binary-prefixes) option.