I'm confused when it comes to the frequency you have to defend against a bet.

For example he bets a street pot size . In this spot you could say that you need to defend 50% of the time , so he doesn't profit , but i also see people saying that you only need to call and win more then 33% , according to your pot odds.

Obviously , calling 33 or 50% of the time is not the same thing , and one of these options makes us lose money. Why don't poker players only stick with one of these concepts (the one that is more accurate when it comes to making + ev choices)? When do you decide to look at the hand through your pot odds point of view , and when through your opponents?