Note to moderators: if this posts is againsts your policy I will give up and I will read these policies :)
Otherwise just I will be happy with private answers.
Thank you!
This is because they are typically situations where a big company that can afford to pay ANY price for the domain decides they want it. They aren't going to limit themselves to domains listed for sale. Pick any random name that Monopolist X wants for some reason, and it's more likely than not that it doesn't happen to be listed for sale. They make an offer the seller can't refuse, and one of the conditions of sale is non-disclosure.
So, there are two strikes against ever discovering the top prices:
1. Unliklihood that the domain was even listed for sale. The buyer contacts the seller privately, negotiates a deal, and insists on secrecy.
2. Non-discolsure requirements imposed by the buyer, in EITHER case. (That is, they will INSIST on it if it is a private sale, and will still do this - if possible - when they buy a domain listed for sale.)