Hi Susan
Good question! There is a difference and it has, I believe to do with your strength and degree of openness on the day.
The more open your shoulders, elbows and wrists are, the more surface area or contact you can have with the floor. This means the more you can push through all these points and the more you can build strength. You may find that over time that as your strength improves that your hands will be flatter on the floor.
I think it is very hard to have that firm solid contact through palms (to build strength in fingers and the whole hand) if the shoulders are not even contacting the floor. When you can spread the force by creating the biggest platform against which to lift your legs and body then you will naturally and preferentially work with flat hands. Of course there may be injury that may prevent this.
I would love to hear other opinions.
I have noticed students who cannot get their legs off the ground much at all, may even unconsciously cup their hands to raise themselves just that little bit.
It is my experience that when I am feeling less strong (in my hands, or in my arms, or even in my core/body) that I notice the effort is sometimes directed through cupped hands. This I noticed has happened on occasion after a break from yoga.
How about you?
Namaste
Gabrielle