What is the reason for the difference in fatwa between one who forgot the middle Tashahhud and remembered before standing, and one who forgot Rukūʿ and remembered after prostrating?

Whoever forgets the first tashahhud (middle tashahhud) falls into one of these situations: 1. If he has not yet stood up straight: he must return (to the tashahhud). 2. If he has stood up straight: it is disliked for him to return; rather, he should proceed and perform two prostrations of forgetfulness (sujud as-sahw) before the taslim. 3. If he has begun reciting: it is forbidden for him to return, and he should perform prostrations of forgetfulness. If someone intentionally returns to the tashahhud after having stood up straight, his prayer is invalidated. If one forgets the first tashahhud and remembers it before standing up straight, he should return to the tashahhud while sitting and does not need to perform prostrations of forgetfulness. The Messenger of Allah (peace and blessings be upon him) said: "If one of you stands up after two rak'ahs and has not yet stood up straight, he should sit down. But if he has stood up straight, he should not sit down, but rather perform two prostrations of forgetfulness."

As for someone who forgets the bowing (ruku') and then remembers it during prostration, he should rise from his prostration until he stands upright, and then he should bow. Similarly, if he omits a prostration, he should return to a sitting position to perform the prostration from that sitting position.