The Mishna on our daf teaches:
One who was to the east of his home when began, and he had said to his son before: Establish an eiruv for me to the west; or if he was to the west of his home and he had said to his son: Establish an eiruv for me to the east, the is as follows: If there is a distance of two thousand cubits from his current location to his house, and the distance to his eiruv is greater than this, he is permitted to walk to his house, and from there he may walk two thousand cubits in every direction, but it is prohibited for him to walk to the spot where his son had deposited his eiruv.
So if he is within 2,000 amot of his house, but the eiruv is further away than 2,000 amot, he can go to his house, but not to his eiruv. Similarly, if he is within 2,000 amot of his eiruv, but his house is further away than 2,000 amot, he can go to his eiruv, but not to his house.
The Gemara opens with the assumption that the man, his house and his eiruv are in a straight line, creating the situation where the man can walk to his house, but cannot continue past his house to the eiruv. In effect, since he is not within 2,000 amot of the eiruv that was established on his behalf, the eiruv cannot take effect at all. He is, therefore, limited to the 2,000-ama radius around him. Since his house is within that radius, he can go there – but no further.
The difficulty with this interpretation of the Mishna is that the second case is hard to understand. If the man, his house and his eiruv are on a straight line, how can he possibly be closer to his eiruv than to his house?
Rava bar Rav Sheila suggests a different way to understand this case: If the man is to the east of his house and the eiruv is placed to the west of the house, but rather than being on a straight line, they are at an angle from one another, we can have a situation where the man is actually closer to his eiruv than to his house. In such a case the eiruv will take effect and the individual will be able to walk to his eiruv, and, indeed, 2,000 amot beyond his eiruv, as well.