I'm confused by the bone density t score?

Relates to max dense: T score compares patient to age/sex/race matched individuals at age 32 (or so)-peak bone density. Relates to statistcs, bell shaped curve. Nl is (-)1 standard deviation and better (top 80+%). Low bone mass (osteopenia) (-)1.0-(-) 2.5 std dev (about #5-20 in a classof 100); osteoporosis (-)2.5 or more (bottom of the class). These vary with age, sex, race. Z score compares pt to same age, sex, race.