Air miles and land miles use different units of measurement. Unlike a driving route, a flight path consists of mostly straight lines with little deviation from where you intend on going.
In the United States, you can calculate air miles in nautical miles, while you calculate land miles by statute miles. Other countries use nautical miles based on lines of latitude. There are 1.15 statute miles in 1 nautical mile.
The distance between two points is less when measuring by air miles because air travel allows you to go in a straight line.
When driving, you follow roads that constantly turn and go around obstacles such as lakes or mountains. It is rare to find routes that take you in a straight line from departure to destination.
To travel from northern Illinois to northern Michigan, you need to drive south to get around Lake Michigan and then north to reach to your destination.
Traveling directly from an airport in northern Illinois to Michigan can save at least 50 miles compared to driving because you can fly directly over the lake.
