Dedupe is missing matching contacts when the threshold is set close to the threshold for the match

I have two test contacts (amongst many others) and a fuzzy dedupe rule matching on 5 things - firstname, lastname, street address, postcode and membership number (custom data field). My rule is set so that each one has a weight of 7 and firstname lastname and address all have limited lengths.

The two test contacts match on three things, so if I set the threshold at 20, I should get a match with a threshold of 21, but I don't. However, if I set the threshold to 14, I get the match, correctly marked as being at threshold of 21 and lots of others at lower levels.

The higher threshold of 20 somehow stops the match from showing up, even though it's there at 21.