The pressure in your arm is not unrelated to or disconnected from the pressure in your legs. They are directly correlated. If the pressure in your legs goes up, so will the pressure in your arm, it’s the same system. The pressure in the entire system is what the arm measurement is supposed to show, that’s why they’re taking it. They usually don’t care about your arm specifically, they are trying to understand the whole system, including the pressure in your legs, which will be reflected in that same arm measurement. That’s the whole point, otherwise it wouldn’t be a good measurement.