Does an increase in rushing volume (attempts) among the NFL's leading rushers lead to a decline in per-carry efficiency (Yards Per Attempt), and how do factors like player availability, single-play outliers, and goal-line usage influence this apparent relationship?

Two sample t-test with pooled variance

Does an increase in rushing volume (attempts) among the NFL's leading rushers lead to a decline in per-carry efficiency (Yards Per Attempt), and how do factors like player availability, single-play outliers, and goal-line usage influence this apparent relationship?