They said that they have big investors backing this project and ready to buy all tokens and that's why there is only 20% of total supply for public sale. So I think there are a big investors behind the sense who brought a big percentage for some privileges and good discount.
I'm not sure but 20% for public sale is a normal amount for an ICO while there is a big percentage in the private sales, it served for a bigger investors which hold the coins or tokens for so many years.
20% is absolutely not normal or common, it's usually the other way around with like atleast 50-60 being available there for the public sale to the people