I gotta agree with Terry on this one. If it hasn't always done this, it concerns me even more. I'd be looking at several things...
- Could a flap be bent?
- Is it possible one side of the steel weldment that deploys the flaps could be cracked? (I'm assuming the 6 is made similarly to the 7 in this regard)
- Is a hardware component loose or damaged?
There may be other causes that I'm missing, but I'd want to find that gremlin before much longer. It may be just a quirk that you live with, but if it's the start of a much bigger problem, then flap asymmetry could lead to a very bad day.