I've asked that same question and argued the same argument but until some expert comes in and revamps the findings we are expected to accept the information as accurate.
My DD was counting before age one. She was fascinated with her grandmother's shirts that almost always had buttons on them so she would place her finger on each button and my mom would count them out. Simple enough and we didn't think anything of it, but while changing her one day she started counting. (Changing time tends to be when I got a lot of surprises. Besides her counting, I also found out she knew her left from her right when I was putting her socks on her.) By age two she was able to do basic addition and subtraction.
And for the ABCs, DD knew all of them by 9 months. Again, a fluke, a friend's toddler was over and I pulled out some little ABC books for him. DD was intrigued and after he left she would pick up each book and ask "What's this?" and "What's that?". Within 3 months she had learned all of her ABCs, but as for singing the song ... we didn't really get around to it for another year or so. I figured she already knew all the letters so it wasn't really important for her to know order and she would eventually learn it. Didn't take her long once she heard it.
So imagine how ridiculous the lists seemed to me. I thought for sure the milestones were not even closely right. I was clearly in the middle of GT denial. LOL
