This is what they want you to think. They will probably tell you this. The real answer is that most people scheduling a ride simply don't know better, so Lyft can pocket the difference. Also, they're probably hedging against a sudden driver shortage so that if there's a small dip in driver supply it makes up for it, and if fares increase beyond what they quoted you, they'll either rip you off with a higher fare, or they'll simply not get you a ride and tell you to go pound sand.