Because, and I say this as someone with absolutely no experience in the area, let's assume Arsenal have a valuation (we don't know this number) of their player, and we want said player. We have bid 30 mill, Arsenal say "nah, you're alright thanks but we'll take 40 million". Arsenal are more than likely not naming the price that they would sell him for. It's probably somewhere in between those two numbers. It's just a standard negotiation tactic.