Because he's played and delivered in the Premiership. David Silva may not even know a lick of English. Silva could take a few seasons to settle and many, many foreign players like him have failed to settle in the Premiership. Milner on the other hand has years of experience playing in this league for multiple clubs in multiple formations and positions, that's the premium and for a player of his talent, age and potential is easily worth another 5-6 million.
Also Milner will be a 'home-grown player' with each club needing to reach an amount of these 'home-grown players'. They don't necessarily need to be English but they need to have come through an English clubs academy.