I think the reasoning is that at the forming stage everyone is keen to create a group, so things are moving forwards quickly. The storming stage then means there is a pause where everyone is jostling for position.
I hate this type of question. All stages have to be gone through and so they eventually all contribute to the working effectiveness of the group – even the storming stage. Whether forming or storming has the lesser effectiveness seems to me to be very subjective.
All I can say is not to try not to worry about it. There will always be poor/ambiguous questions but everyone is in the same boat.