The primary reason some people are asymptomatic is that they got a small dose of cov-19
This is a main reason the illness is so different in people. It apparently also depends a lot on the initial viral load that infected you. Your own body’s immune response to it doesn’t help.
I’m late 30s, very fit, healthy, no vitamin D deficiency, etc. Didn't get a fever, never most my sense of smell, never had a dry cough.
This was still worse than a bad flu for me. My running was affected for many months afterwards.