That’s an exact amount of force on your entire body when someone pushes you, it doesn’t change. When you expose more surface area of your body to water the force increases with it. On one foot you have maybe 2% of your body exposed and what amount of force the water can exert on that small area vs your entire body weight bing planted that it has to shift. Once you get on all fours you still have the same weight, but shitloads more force from the water enacting on it. Plus, now that you have your weight distributed on multiple points it’s far less force required on each individual contact point to move them, and take you away with the water.