Offers put during negotiations are often 'without prejudice' and, if they are seen to be genuine offers as part of a negotiation process agreed by the parties then they are likely to be upheld as made under privilege and, as such, cannot be referred to in later proceedings.
This is because the courts want parties in dispute to be able to negotiate, prior to raising the matter to a court of law, without being fearful that their discussions and offers will be used against them at a later date.
If it wasn't for this reasonable convention we'd have less disputes settled in negotiation and far more taken to court.
It's also the case that an awful lot of correspondence in matters of dispute are marked as being 'without prejudice' when they actually aren't. Just because an offer says that it's without prejudice doesn't mean it is.