Practice Swing Speed Versus Contact Speed

[Mayday16]

I’m using a SC200 Plus and can regularly register swing speeds in the 70s using the practice swing speed mode. When I change to the practice mode and start hitting balls that drops into the 50s. I know your loose practice swing when not worrying about a ball can be faster but I never seem to even luck into hitting one in the 70s when hitting a ball. 
 

I have noticed when I hit it low or thin the distance and swing speed are higher. Can the contact of the ball effect swing speed? Could it be the way the SC200 works that I am actually swinging in the 70s but due to bad contact it registers less?

[iacas]

With what club?

Do you have a video of your swing?