(Mishnah): If one was in the east and told his son to be Me'arev for him in the west, or vice-versa:

If he is [at most] 2000 Amos from his house and further than this from his Eruv, he is permitted to [go to] his house and forbidden to [go to] his Eruv;

If he is 2000 Amos from his Eruv and further than this from his house, he is permitted to his Eruv and forbidden to his house. (This will be explained.)

If one put an Eruv in the Ibur of the city, it has no effect.

If one put an Eruv outside the Techum (Ibur of the city), even one Amah outside, whatever he gains [in that direction] he loses [in the opposite direction].

(Gemara): Due to a question that Bar Kapara had, [we must explain our Mishnah to mean that he was] to the east of his son or to the west of his son.

(The Mishnah connotes that he was east of his house, and told his son to be Me'arev for him in the west, or vice-versa. I.e. his house is in the middle.) Granted, if he is within 2000 Amos of his house and more than this from his Eruv, he is permitted to his house and forbidden to his Eruv;

Question: [How can he be] within 2000 Amos of his Eruv and more than this from his house, that he is permitted to his Eruv and forbidden to his house?! (He is closer to his house than to his Eruv! Rather, our Mishnah means that he was] to the east or west of his son.)