"Electrically common" means any points in a circuit that have 0 voltage potential difference between them. So even if there's a 250V difference in potential between point A and ground, and a 250V difference between point B and ground, there is 0 difference in potential between points A & B; therefore they are electrically common.
If there was a 250V difference in potential between point A and ground and a 255V difference between point B and ground, there would be a 5V potential difference between points A & B, and they would not be common to each other.
If there was a 250V difference in potential between point A and ground and a 255V difference between point B and ground, there would be a 5V potential difference between points A & B, and they would not be common to each other.