Wrong odometer reading on the car, not actual mileage?

I bought a car which is actual mileage that show on the clean title. On clean title efficiency show 185000 miles, on the car is show 113000 miles. What happen to it? How do these guy pull the miles off? it's illegal?
Second, If i drove 20,000 miles on the car, does it add to the actual mileage like 185000 + 20000= 205000 miles?
 
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