Some people are able to keep skinny better than others...
I'm on my feet 9 hours a day 5 days a week at the shop. Very little cardio but a decent amount of muscle work and steady movement. 5'10 160 lbs give or take a full belly. I've been the same size since high school.
That being said, I'm not in particularly "good" health. My cholesterol is a bit high, as is my BP. I'm 29 years old. Smoking, drinking, and a generally [censored] poor diet do wonders. My saving grace is that I don't have a huge appetite and can be comfortable on one meal a day, plus a light snack in the evening. Portion control is as much a factor as what's in those portions.
My father: he's been a radio talent for 35 years. Talk about a desk job. Entertaining yes, active no. However, he plays intramural soccer twice a week and does cardio when he can. Eats well, doesn't smoke rarely drinks more than a glass of wine. 65 going on 30.