The Difference Between Bosporus. Panticapaeum Ae20 Ae21

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They are the same type, yes, just slightly different in size. (In fact the 21mm coin weighs slightly less than the 20mm one.)

For many ancient Greek (and not just Greek) coin types, numismatists are not quite sure what the denomination actually is, so (at least for Greek coins) the convention is to represent it by an abbreviation for the metal followed by the size - so, yes, AE20 is a 20 mm bronze and AE21 is a 21 mm bronze.

Presumably some internal system at NGC treated it as a denomination distinction (which it technically is, just not enough of one to be actually important), which triggered separate entries in the population guide.

Note that you couldn't just assume that the same design in different sizes is the same type, because there are legitimately different ancient coin types/denominations that have the same design and are distinguished by size (though of course by larger differences in size than this) - so edge cases like this would have to be fixed manually, and I can see why NGC didn't bother.

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