If I knew lefty was about to jump to 4♥, I would bid 2♦. But holding 19 points, I would worry that it could actually be our hand. Does 1NT risk missing game? I don't really think so. My range is 16-19 in this situation, so that seems close enough. Look at it another way - if partner has ♦Q and the ♣AJ, he will invite game and we will most probably make it, as we will opposite a lot of seven counts. However, if pard just has a balanced pile, I would much rather play in 1NT than 2NT, since 1NT will pretty much always have some play, whereas 2NT needs quite a bit more from partner.