They just guess. I wouldn't take any notice of that site. I believe they have Nketiah at a higher value than Mateta, for exampleThat is what I understood, but the inaccuracy of what is out there in what is supposed to be confidential discussions is the context.
On which, Transfermarkt only has a sale to Liverpool as the sole rumour and (get this) assesses the chance of transfer at 23%.
What variables does it apply to arrive at such an accurate prediction? I would confidently state that this figure can only be accurate to a chance of 62%, even with a +-10% margin.