This always surprised me as well. Also, there is never any mention in the movie about the fact that the names all start with the same letter.. minus Allie.
shareDennis and Doris were coincidental (hopefully, or it may explain a few things), but likely sparked the trend. Ally may have been an unexpected late-in-life surprise, and the "rule" may have seemed outdated by then, or perhaps it's just her nickname. Or, a different father or adopted?
It's probably just a red herring, and here we are....
Life is tough, but it's tougher when you're stupid.