I want to purchase a flow hive super to place on top of my existing hive body and i have this questions?
If I am reading your question correctly the reason that fewer flow frames fit into a standard box is that they are wider then a standard frame. 6 go into an 8 frame box and 7 into a 10 frame box. The box is a standard box though not a custom sized box.
There can only be specific combinations of boxes, standard frames and flow frames. Here is the best explaination Building the new hives when they arrive