A quality scope mount is only one half of the equation for repeatable zero. Also need a rail that is properly machined. I have come across different flattop receivers that have had some errors in machining, and this isn't specific to any one brand, even seen it with Colt. If the rail is not uniformly machined along its length, you are going to have problems with a repeatable zero.
It should be noted that no matter how tight you attach the mount to a rail, it will move on the rail to what ever amount the recoil lug allows in the rail cutout. I have seen mounts using thumb screws that were really cranked down, move rearward on the rail, [b]against the recoil[/b], yet once the screws loosened up a bit, allowed the mount to move forward with recoil. However, with throw levers, your tension will stay even, and usually after firing about five rounds, the mount will settle into its happy spot, and generally not move from then on until being removed and replaced.