Looking at the Revision history for this property and other ISFDB ID that I find here {1233 1234 1235 1274}, it seems to me that you made a mistake on this one 2021-03-02. "... Created claim: number of recoveries (P8010): 391,952 ...".
By the way, does this report by Mix'n'match represent the number of Wikidata records where at least one value for ISFDB publication ID is assigned? So that comparison 391,952 and 661,563 shows loosely speaking a 60% completeness (not a magnitude I would have expected) of Wikidata with respect to ISFDB publication records?
My talk page here suggests that I don't use it, but I will see anything there --as well as here if the lag is not too long.
(ISFDB content editor) --~~~~