Our driveway is at an incline so I also noticed the same thing as you when the electric brake is engaged during parking. I also noticed same thing on my wife's Odyssey minivan parking foot brake. On that it moved a bit more than the GLC. It was a bit unnerving when we first bought the minivan, but we got used to it. My Civic Si has a handbrake and like you said, car doesn't move once it's fully up. I'm still trying to get used having electric brake. There's something to be said hearing that clicking noise when engaging manual brake that gives a sense of security knowing that the brake is engaged when parked. Just like everything else, I'm sure I will get used to it soon enough.