You may be using the cart to support some of your weight. That takes a good deal of strain off of your muscles which in turn causes you to use less oxygen.

It is wheeling you along which takes the weight off and a place to put your oxygen which again is less weight then when you were walking carrying your body weight and the tank or machine. I shop with someone else. Usually, my son or daughter they push the cart with the groceries. Aside from my o2 tank the only things in my cart are bread, and a few other light things because the more you add to the cart the more it weighs which is defeating the purpose. I wish you well on your journey.