Maybe 65 should be explained as said like "five and threesh" where "threesh" is implicitly/shortened for "three times 20". So we say: 5 + 3 x 20, but in a terse encoding.

Yes, 55 is "half threesh" in the same way, that is we say 5 + half 3 x 20, but half 3 == 2.5, of course!