As the spouse of a doctor, this made me giggle. If her practice had 90% margins, my life would be a lot different than it is. Her actual net margin, as in what she brings home after paying all the bills, is closer to 15%.
Put another way, the rent, employee pay, supplies, malpractice insurance, etc. etc. etc. add up to about 85% of her gross revenue. That 15% difference is how much she personally makes.