Why didn't Anne bet on the red horse with Diana? Why did Miss Barry like Anne so much?

From the text:

Diana got so excited that she offered to bet me ten cents that the red horse would win. I didn’t believe he would, but I refused to bet, because I wanted to tell Mrs. Allan all about everything, and I felt sure it wouldn’t do to tell her that. It’s always wrong to do anything you can’t tell the minister’s wife. It’s as good as an extra conscience to have a minister’s wife for your friend. And I was very glad I didn’t bet, because the red horse did win, and I would have lost ten cents.