So... this happened

I need to write this down somewhere, so I can share how confused it made me.

G and I were at the library and I left her reading to herself in the kids area while I wandered a bit away looking for some books for myself (she was within maybe five feet of me). I heard her say "No! That's mine!" and so I came back and there is a little boy, about a head taller then her STANDING on the book she's reading - which is currently on her lap. He gets off, but he keeps trying to stomp on her foot. G is upset (obviously) and saying "No! Go away!" and (her favourite new phrase) "Don't look at me!"
  The boy says "Oops! Sorry!" as he continues to try to step on her. So I intervene. I say "No, no, let's not step on her foot." "Oops! Sorry!"
So he grabs her toy (that was in her hand) away, and, of course, G screams "NO!" in her angry-toddler-that's-mine voice. "Oops! Sorry!"

Enter his mother.
She's kind. She pulls her son away, but then, we have the following conversation:

Her: How old is she?
Me: Almost three.
Her: Oh, when's her birthday?
Me: She'll be three in August.
Her: Wow, she's a couple months older then him! He's tall for his age.
Me: Yeah.
(The little boy tries again to take away G's toy)
Her: Sorry! He thinks that she's littler then him, so he can beat her up.
(to her son) She's older then you, so you can't take things from her.
Me: uhm...
Her: He doesn't know she's older then him. What size does she wear? 2?
Me: Yeah, mostly.
Her: She's older then you, sweetie, don't pick on her!
(They leave)

So... it would be okay for him to pick on her if she was younger then him? I do not understand. I guess she was just surprised that G is almost 3 since she's so small. But seriously! The reason not to take a kid's toy or step on them is because you don't step on another kid regardless of their age.