Alternate powers largely mean that you can only use one of the powers at a time. So if I had Blast 10 (Alternate Power: Flight 10) I can either fire a blast or fly at any given time, but I can't actually do both. It's an either or equation - I can't mix and match the levels of the power used.
If the same power was a dynamic array, then I could choose to lower my Blast to 8 in order to use the other power in the array at 2 - So I can fire a weaker blast, but fly at a speed well below my maximum.