Nowhere in the
rules of the game does it say "must be guaranteed to score a try". The rule actually says -
"the Referee may award a penalty try if, in his opinion, a try would have been scored but for the unfair play of the defending team."
It is solely the referee's opinion whether a try would have been scored but for the act of unfair play. If he/she felt it would have been scored, its a penalty try. If it isn't, it would most likely be judged a professional foul and the player would be off to the sin bin.