I visit family in other states and I get comments like “I can’t believe you are so thin.” For context I am a healthy weight and I eat what I consider a reasonable diet. I sit and smile while I watch them drink soda and eat pure sugar and salt. I don’t care about your life choices but don’t act surprised by someone that’s a normal weight.
A BMI calculator will just use height and weight - if you have a lot of muscle bulk you’ll get a higher BMI result even if you’re lean. Conversely, some people are “thin on the outside, fat on the inside”, ie they carry a lot of visceral fat around their organs. So while BMI can be useful, it’s not perfect.
A skinny-fat build can be deceptive. I thought I just had big bones for a long time, but I was easily able to go from 205 to 185 lb without really looking any different. That dropped me from overweight BMI to normal BMI