Official CDC list of Covid-19 symptoms that are commonly caused by lots of other things.
Fever or chills,
muscle or body aches,
congestion or runny nose,
nausea or vomiting,
Most of these are not yes/no symptoms but have wide ranges that might not even be noticeable unless pointed out. And some of them afflict a lot of us regularly. Most people cough from time to time or is it just "clearing your throat" Muscle aches? Fatigue? I suffer fatigue every night when I go to bed. Or is fatigue not the same as being tired?
There are ways to calibrate symptom reporting. For instance using interviewers that don't know whether the patient being interviewed is positive or negative or is even a suspected contact. Add in randomly selected people. Gather statistics on the differences. Calibrate interviewers as each one may inadvertently, and innocently influence answers.
It's astonishing how non blinded testing can skew things. Facilitated Communication being a classic.