There lies IMO one of the big problems regarding tightening scope mounts, the "screwdriver".

When a screwdriver is selected which has a blade broad enough to engage both sides of the slot on the rings, you could end up with a screwdriver powerfull enough that it is possible to badly over-torque the mount clamp system, and something is highly likely to give.
In this case it's the tip of the clamping claw.
Once that tip is deformed, the claw is not able to draw the scope mount down tight onto the receiver, and the result will be just as you describe, where the screw is still tight but the scope base is able to move.

(Edited to add.........
Of course it is still possible that the metal was too soft to start with and would have deformed anyway).

I used a screwdriver once for tightening the clamp nut on a set of scope rings.
That was back in 1979 and I haven't used a screwdriver for that purpose again.
IMO it's better to use a coin of the correct size.

Our Oz 10 cent piece is just a tad smaller in dia than 1 inch and it's thickness fits the slot nicely.
It does the job well, and will get the nut plenty tight enough.
You would really have to excell yourself though to over tighten Ruger rings with a coin.
With a screwdriver however, it is not at all difficult to over-torque those mounts.