I've run across a few rails and rail to receiver fit ups that don't come out level. Setting the rail on the table as shown isn't a good method check it. This is due to the radius cuts on the bottom. The bottom edges that rest on the table could be off, even if the radius cut and holes are true to the top side. Putting the rail on the rifle and measuring it to the receiver is a better 'at home' way.
Is your receiver a factor or aftermarket custom? Its very common to see tangs and bolt race ways that are not level. Make sure your level on the receiver is bearing flat. Sometimes it can get hung up in tight fitting areas. I find the tang is the most consistent point to measure from. However, at the end of the day, the tall target test will tell you if the reticle is level to the bore. I'd recommend performing this test with a few shots prior to buying a new rail.
Another thought...when mounting the rail to the gun, set the rail on top and lightly install the front two screws. Just enough pressure to seat the rail. Check the level front to back and see if there is a gap between the back of the rail and receiver. Torque down the front two screws and check again. Install the back two screws and check again. If everything reads the same, then it