I'm really trying to think of a single reason for that to be relevant to Berge.
I can only think that unproven is worse than proven to be slightly above average sometimes in the Championship.
A group of talent ID professionals with specialist skills assessed Bellingham's current and potential ability, decided he was worth acquiring him and took the steps necessary to buy.
Likewise a similar group at SUFC assessed Berge's current ability and his ceiling in 2020 and decided to sign him. Right now his value is affected by his recent return to full fitness and the division we're in but the fact that he has 3 years left on his contract and no being under financial pressure means his fee remains high.
Groups of scouts all over Europe will have assessed Berge recently and the only reason he's not an attractive signing is because his fee is a lot higher than his value. Some clubs take those risks, but they probably don't shop in the Championship. That doesn't mean he's not a quality player. It certainly doesn't mean we should sell him for whatever we can get.
The "unproven / proven" stuff is absolute bollocks and peddled by plenty without a clue including some managers.
'Proven' is supposed to mean done it before, therefore can do it again but in reality means easy to find, usually expensive, peaked, under pressure to deliver. You don't need scouts to find proven players, any daft sod can do it and it's hard to do it well.
'Unproven' is supposed to mean something negative. Hasn't done it before so why should we believe he can do it all etc. This is where scouts make all the difference, they find tomorrow's talent while the clubs obsessed with
proven players are losing money and buying shit.
With no disrespect intended, if your assessment of Berge in the Premier League is "anonymous" and your assessment of Berge in the Championship is "sometimes quite good" you're either unable to spot a player, blinded by how much he cost us in the price, or a bit of both.