What you're seeing is people who match into dermatology and radiology programs which have a built-in preliminary medicine year. Take the Mayo Clinic, for example. They offer 2 types of dermatology positions: categorical, which means you have to do a preliminary year of medicine at the Mayo Clinic followed by 3 years of dermatology there; or advanced, which means you can do a preliminary year of medicine, surgery, pediatrics, transitional, family practice, OB/GYN, or emergency medicine at any hospital you choose before your 3 years of dermatology at Mayo. The hassle is that you have to apply and interview for both prelim and advanced programs.
Radiology was actually a 4 year residency (without an prelim year) not too long ago, until rads programs decided to make a year of broad-based medical training mandatory.
AV