technically not true. is just that after AD is DD and DXO DD jobs are very few and also very niche. they might open the job to outside if they need new skillsets that currently mindef do not have or updated, i.e. data science kind of jobs.

thats probably why you will only see AD as the highest and typically DD is taken up by those in the AD positions for a long time. you can still apply DD, but in the end, best men wins.