Yes, and as Mayo clinic above lists also polips and a fracture, i.e. unsurprisingly if the nose is clogged or its inner lining is altered, you loose your smell,

Yes, and as Mayo clinic above lists also polips and a fracture, i.e. unsurprisingly if the nose is clogged or its inner lining is altered, you loose your smell, but I would have thought that the sample was excluding these, either because they can be easily diagnosed (polips or fracture) or - as you say - because typical cold (or allergy) symptoms are not usually connected to Covid-19. All in all the causes are listed under three main categories: 1. Problems with the inner lining of your nose 2. Obstructions of your nasal passages 3. Damage to your brain or nerves If - as I think - the first two are excluded, the 20% of non-positives but suffereing from anosmia seems very high to me. Also, as it was reported anecdotally: https://news.ycombinator.com/item?id=24664925 https://news.ycombinator.com/item?id=24664925 the anosmia from Covid-19 seems to be much stronger than thoe connected to cold and similar.