It's bloody ridiculous. The USA had a fairly simple, clear, & easily understandable system of designating military aircraft, AFVs, etc., I think clearer than anyone else's - & for no good reason I can see it's thrown it away, & started allocating random numbers. It started with F-35, where the X-plane number, for an experimental design, was carried over to an operational designation, contrary to the rules of the system, but it's also happened to different marks within a type. The old A, B, C system has gone, & E now means a radically different variant, with block numbers being used for what would once have been different marks. That seems to have begun with F-5E, carried over to F-18E (should probably have been F-24) for political reasons (honest, it's not a new aeroplane!), & now, with no formal change, it's become the rule. How that will be fitted into F-35A, B & C will be interesting to see.
F-35A/B/C should probably have been F-25, F-26 & probably F-27.