Imagine trying to balance a ruler with 2 pieces of string. (close to the center of the ruler)
Now let the string go further towards each side. The further the piece of string move outwards, the easier it becomes to balance.
Swap Lift for the string, and wings for the ruler.
More lift is produced by the flaps near the center. Less on the outsides. When no flaps, it's a lot more stable.
Well, that's what i was told 2 weeks ago. I'm now having second thoughts...