It takes about 285000 skyward miles to redeem a first class ticket from Asia to America but it only takes 200000 Alaskan miles to redeem that award.
Airline miles are a currency, just like real money. Some miles have a larger worth then others.
Just compare the earnings:
A flight with Emirates from DXB to LHR oneway in first class earns 7500 Skywards miles, but only about 5200 Alaska Mileage miles.