I have Calibre 64bit 6.28.0 installed. PMPC shows this as being current, but Calibre 6.28.1 is the current download on the Calibre website.
According to the Calibre website, 6.28.1 was officially released on 10/7, so it may take PMPC a little time to catch up.
https://calibre-ebook.com/whats-new