Hand texture plays a pretty key role as well. You can discount medium-strong hands (unless your opponent over-plays them) and draws. Generally when a card/flop hits that changes the texture of the hand someone will pause to re-consider what to do about it. How much to bluff or how much to extract.

Also live, many weak players will under-play very strong hands like sets adn top two when a draw hits. So the insta-bet is much more likely to be a bluff or over-played hand from such a player.