B&S Model 44 V-Twin engines w/ same displacement but different horsepower - ???

Hello. I have a John Deere LA120 riding mower with a Briggs Model 44 V-Twin that is rated at 21 horsepower. I've always felt like the mower could use a little bit more power. Just for kicks, I was doing some poking around online tonight and noticed that the next "bigger" John Deere in this series was the LA140 that was rated at 23 horsepower. Digging a little deeper, I noticed that both engines have a displacement of 724 cc. And then I noticed that they have identical bore and stroke dimensions as well.

Does anyone know what the actual difference is between these engines? Is the 21 horsepower version just have slightly lower compression, different carbs, or maybe just detuned in some way? It's obvious where I'm going with this - I want to unleash those 2 extra mystery ponies!!

B&S Model 44 V-Twin engines w/ same displacement but different horsepower - ???