The Tautology Problem and Fitness as a Propensity

Suppose we define ’fitness’ as,

Fitness of an individual or type of individual as the actual number of offspring produced.

Likewise, suppose the average number of offspring of type x is greater than the average number of offspring of type y.

Why did x on average leave more offspring than y? Because x is fitter than y.

But, given the above definition,

Why did x on average leave more offspring than y? Because x on average left more offspring than y.

This is no explanation -- it seems to be a "tautology." Mills and Beatty write,

"The whole idea of setting up empirical investigations to deter- mine whether fitness differences are correlated with actual descendent contribution differences seems absurd, given the above definitions of fitness."

How does the Propensity Interpretation of Fitness resolve this problem?