Because Avmed interprets and applies the rules in whatever way will be the most inconvenient, costly and intrusive for you. Of course, it's dressed up in safety rhetoric: The timing is determined by reference to the date on which the examination or whatever happened that produced evidence of your fitness occurred, not the date on which Avmed gets around to assessing that evidence and issuing the certificate, because you could suffer some condition 6 months and 1 day after the examination and bring down a Jumbo Jet. To Avmed, those periods are holy writ. (To qualified medical practitioners and specialists the periods are arbitrary, anachronistic and not justified on the basis of evidence and risk, but Avmed knows better.)