We have two Britax pioneer car seats for our daughters. We've tried two different cars but have the same problem. When we tighten the straps, the seat part of the car seat lifts up off the... seat in the car. That's slightly confusing, I know.
We use the stabilizing harness (hooked to the back of the seat) and the anchor latches... But the anchor latches only tighten on one side (so it's the same physical strap connecting both anchors).
Our other car seats have been super snug and don't move at all when we tighten the straps. These move several inches in either direction if we push on the base side to side.