If you take a random directed graph and separate the nodes in two sets A and B you would expect* the arrow from A to B to be about the same as those from B to A

If you take a random directed graph and separate the nodes in two sets A and B you would expect* the arrow from A to B to be about the same as those from B to A regardless of the relative size. * on average on all partitions with |A| \le |B|