What does NORMAL child development look like?

I really think the idea of "normal" is not as useful as I once thought it was. When I first started teaching I based all of my ideas about normal child development on the assessment tools I used to test for developmental delays. The test we use is the Speed DIAL. (Developmental Indicators for the Assessment of Learning.) It is supposed to scale sores by developmental age down to the month.
Twelve or thirteen years ago my 4 year old students usually scored fairly low but not below the "cut score" that indicated a delay.

Having my own kids made me stop thinking about normal development so much. Every kid is so different, unique and brilliant in his or her own way. Also, having my own kids and teaching a group of kids for two years has made me raise my expectations for what kids are capable of in language and cognitive development.

Most of my students from last year scored at about a 6 year old level according to the DIAL.
One of them didn't. She scored at a 3 year old level. If she hadn't been in my class last year I wouldn't think twice about her development but, because all of the students from last year scored higher I am thinking she may have a delay in language development. Her mother and I agreed to do a child study. I just don't want her to to be unsupported if she needs additional services. She seems to have extraordinary expressive language (what she says) but delayed receptive language. (what she understands and retains)

She only seems to be delayed within the context of my classroom, not within the broader definition of student delays. There are so many lenses to look at her development through. I am not sure the "normal" lens is the best to use to understand her developmental patterns.