How do you not know that I know that from my post? It's a stupid standard.
Because the screen has enough pixels to be high quality. Just coz it's not big enough, doesn't mean it's not high definition.
High definition, as a phrase, just means highly defined. High, in that phrase, has just been stupidly defined as a minimum of 720 pixels across (usually at 16:9 ratio).
But if you have a smaller screen, than it obviously needs less pixels to be highly defined. If it still had 720 pixels across, it would be hyper definition or something.