Why does Liverpool FC find better transfer deals than Manchester United FC? There are many factors I can only guess at - scouting networks, internal politics, personalities, and so on. But there is one thing I'm pretty sure about, which has to do with debt and credibility.
Generally, MUFC runs a much higher debt burden relative to LFC; in soccer terms, LFC barely have any debt at all. This means if I'm a foreign club negotiating with these English giants, I have slightly different tactics - with MUFC, I can always entertain asking for more since they are willing to borrow; with LFC, I have to entertain the possibility of insufficient funds. If you have a reputation of carrying no debt, it's credible when you say you have no money; just open your empty wallet, and keep the credit card hidden.