The reason for this is that the payload of the aircraft cannot actually carry the total pax amount with the standard pax weights. Here we use a standard weight of 83kg per passenger and added with 15kg of bags, and for small size class we assume that 60% of paxes have checked bags, making the total pax+bag total weight of 92 kg. (In some other operations (in real life) they can use actual pax weights (less than 92kg on avg then) and use the full load but here we use the standard model..)
For 110P2/41 the max payload is 1860kg, so it means you can fit "20.2" passengers; rounds to 20.
(won't change the description since the pax weight calculation could change later .. though no plans for it)