EFB hides the process and just gives you the magic figure. Go through a RTOW chart. In flex calculation what role does the OAT have? You go for ETOW and not temperature and for the calculated Flex temp to be valid it has to be higher than OAT that's all(and Tref). By how much is not a condition. Takeoff speeds change even with surface wind. Previously there was a restriction of flex had to be higher by some 7 or 9 degrees to cater for OAT increase during taxi. But now the modified ECAM software gives a warning during TO and automatically sets TOGA thrust if not done in ten seconds or so.
WhiteKnight your question is a doubt but certainly not some Eureka. Flex temperature has been around and certified for decades. All angles have been looked into.