I would find it odd as well (if this was my child). That being said my kids had (and still have) inconsistencies in development, with some things seeming very advanced and then simple things going over their heads. They are both 2e, for whatever it is worth.

The nice thing about number 5 is that you can see it easily in daily life, and talk about it a lot. Like 5 fingers, 5 toes, 5 points on a star, etc.