50.23% is 10.53 percentage points better than 39.7%, but it would be (50.23%-39.7%)/39.7% = a 26.5% improvement.
To understand the difference, think about this: if the Seahawks had a cheat code and converted third downs at twice the rate they did last year (twice 38.32%, or 76.64%), they'd have improved by 100% (100% improvement means doubling), but by 38.32 percentage points.
If you take the Seahawks' 38.32% third-down conversion rate from 2022 and improve it by 10% (that is, by 3.832 percentage points), you get to 42.152%, which would have put them in ninth place. Take last year's rate of 38.32% and improve it by 25% and you get 47.9%, which would have been third in the league.