After my Dad died, my Mom got a Pekingese to keep her company. She had a maid who would come in once a week to help her out, and that lady swore the dog was just going to start talking to her one day.
Here's what I think is going on:
Humans and dogs are both social animals and they both need a great deal of social contact (obviously). Your mom's dog was alone with your mom all day every day. Your mom probably lavished attention on the dog and spoke to it constantly. The dog learned to understand english much better than the average dog.
Your mom's dog was probably also smarter than the average dog, but it was the ability to understand english that made it seem so freakishly smart.
My wife and I had a dog like that when we first got together. I'm pretty sure it was because it was just the 3 of us and we took him everywhere and babied him. None of the dozen or so dogs we have had since has been able to understand us like that first dog. I think we stopped paying so much attention to the dogs when the first kid was born:)
If you have more than one dog, they will spend time socializing with each other and they won't learn english as well as a lone dog in your house.