the message that 1NT conveys is that it denies 2 cards in ♥. On the contrary, dbl might also has 4-4 in minors but it has exactly 2 cards in ♥. Particularly, having a 4 cards in ♠ my partner might wants to defend. Thus, 1nt is likely to mean 3=1-(54) or a more unbalanced hand Thus, rebidding 2 ♥ after 1 NT respond opener has 7cards or a very good 6cards (something like AKQxxx). On the contrary, rebidding 2 ♥ after dbl opener might has a 5cards in ♥, without a stopper, that for some reason he/she does not want to play a contract in minors. I don't know if such a differentiation between dbl and 1NT worths something or if it is completely pointless. I haven't tried it in practice