The Mishna on our daf discusses a case where the person who is slaughtering the Passover sacrifice has intention that the korban be for people who will not be eating from it. Such people include individuals who cannot eat the meat of the sacrifice because they are old or ill, people who had not joined this particular group, or people who were not permitted to eat from the korban, e.g. someone who does not have a brit mila. In such cases, if the intention was just for such people, the korban is no good. If, however, the person thought about people who would eat from the korban, as well as people like the aforementioned, then the sacrifice is valid.
The Gemara asks: From where are these matters, which are not explicitly written in the Torah, derived? The Gemara answers: As the Sages taught with regard to the verse: “And if the household be too little for a lamb, then he and his neighbor who is close to his house shall take one according to the number of the souls; according to every man’s eating you shall make your count for the lamb” (Shmot 12:4). “According to the number of” teaches that the Paschal lamb is slaughtered only for those who have registered for it. Everything is done according to the number of people who have registered before the slaughtering.
I might have thought that if he slaughtered it for those who did not register for it, he would be considered as one who has violated a commandment, but nonetheless the offering would be valid after the fact. Therefore, the Torah teaches this law with the double formulation of “according to the number (bemikhsat)” and “you shall make your count (takhosu)”; the verse repeated it to make this requirement indispensable, so that the offering is disqualified if it is slaughtered for those who did not register for it.
This discussion points to one of the ways in which the Talmudic hermeneutics differ when dealing with issues regarding sacrifices. Generally speaking, when the Torah commands us to perform an act in a specific way, it is understood that if it is not done properly, the act is an invalid one. Regarding sacrifices, however, it is commonplace to find that a single passage may command that a specific action be done, yet if one skips that detail, the sacrifice will remain valid after the fact. Only if there is an extra pasuk – as in our case – or a specific key word, does the Gemara conclude that it is essential for the sacrifice.