I don't agree with "deviation" because, in mathematics, deviation is a measure of difference between a specific value and some other value. You are talking about a value being outside of a range. In statistics, deviation is often used to define a range - for example, if you take a single value such as a mean average, you can define a range by calculating several standard deviations above and below that value. As you already have a range defined, this term is not only gramatically inappropriate, but to anyone with a knowledge of statistics or mathematics it is just confusing.
"Discrepancy" is not appropriate either I'm afraid - it also refers to the difference between two specific numbers, but also implies a mistake.
I'm not sure that you will find a suitable single word for this context. If you are trying to avoid the terms "overweight" and "underweight", you would be better referring to your range by name (for example, BMI, if that is the range you are using) and simply say that someone is "above" or "below" that range. If you must have a single term, perhaps "out-of-range"?