Ben and Vader assume that Luke and Leia are 'strong in the Force' without training.
Stronger than Han or Chewie or Sy Snootles or Cliff Richard.
They don't assume anything... it is a fact that can be seen and sensed. Luke demonstrates his abilities throughout the trilogy. Liea shows her ability when she can sense Luke through the force on Bespin. And nothing in the trilogy explicitly states that they inherited their abilities from their father.
The quote "The force runs strong in your family" can be purely an observation of the facts that can be noticed.
Since you still don't get it... Luke, Leia and Vader all wear blue shirt. An observation of facts would be "Skywalker family wears blue shirts". Now does that explicitly tell you that they were born in blue shirt?