You know, I have heard that too and have never really given it any thought until you posted your question!
I think the answer is that many years ago, young mothers really didn't know how to go about potty training boys properly. Being women, it was easier for them to potty train their own daughters. Nowaday, we have a lot more knowledge at our fingertips with books and videos. Also, women have more knowledge about boys and men, in general. Years ago, their mothers and schools never prepared them for men and sex the way they do today. I remember both my grandmothers knew nothing about sex until after they were married. Menstruation wasn't a subject they were taught or understood fully either until adulthood! When my grandmother got her period she thought she was dying! And, her own mother's explanation was downright strange. My great grandmother just said, "Well, it looks like we have another woman in the family." Great explanation, huh?
Anyway, that is my best guess. You did make me curious to see if it was true or not. Here are a few links to websites that I thought you might be interested in reading.
Potty Training Girls - LoveToKnow Baby Toilet training boys Are girls easier to toilet train than boys? 
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