⚡ Quick Fix (TL;DR)

The Culprit: Two things can change the number without the site changing: the scoring method itself (a measurement was fixed on a known date), and a scan that was blocked by the site's security software, which zeroes several checks and averages a different set of checks.

The Fix: Record the dates scoring changed and only show a numeric change between scans on the same method; otherwise say the scores aren't directly comparable. Detect blocked scans from the stored results and leave those checks out of the comparison. Also note that only the top few fixes are stored, so a fix that disappears may just have been pushed down the list. Word it "no longer listed", not "resolved".