I think you'll find that it was the users rather than the level that was faulty or inaccurate. My DeWalt is spot on.
It's easy to check the horizontal line. Just put the level on a solid surface facing a wall a few meters away. Make a mark on a wall at that LH end of the line. Now rotate the laser to put the middle of the laser line on the mark and check. Then rotate again and check the RH end of the laser line. It's easy to see if the self-leveling is out. Obviously, the further away the wall is, the more that any out-of-level will show up.