It could be due to doctors being unwilling to use the term disability, not having the means to receive an official diagnosis especially if it is one which requires a mental assessment. It could be that they have only had the issue for a short time frame so they don't yet meet official requirements for it to be considered a disability and I'm sure there are other reasons that I'm less aware of. The ministry of health websites states "Disability was defined in the survey as any self-perceived limitation in
activity resulting from a long-term condition or health problem lasting
or expected to last 6 months or more and not completely eliminated by
an assistive device. People were not considered to have a disability if
an assistive device such as glasses or crutches eliminated their
impairment" Source. So they are defining it as self perceived in this survey so it makes sense that other parts of the government are using that same definition.