Miss Maudie made a gesture that implied to her that Jem had matured greatly. Why?

Miss Maudie called Jem over to her house; Scout and Dill accompanied him.  Maudie took the children into her kitchen, where they found one big cake and two little ones.  Scout and Dill each received their own little cake, as was Miss Maudie's custom.  However, Jem was not given his own cake, but was served a piece of the large cake.  This was Miss Maudie's way of acknowledging Jem's maturity; she treated him as she would an adult, rather than the same way she treated Dill and Scout.