I do find that some seats seem to not want to line up with the pan, ie, the screw spacing is right for the holes but when the brackets look right to the edge of the pot, the seat is slightly off centre or at an angle. The screws are normally smaller than the holes, being mostly metric threaded studding, with a selection of holes in the bracket left right front to back. I guess I'm just being too picky, I did use the plastic tapered washer things to centre the screws, but in the end had to settle for the slight misalignment of the brackets and a straight seat option. I think the error is in how the plastic brackets are screwed to the actual least and lid or the geometry of the bracket either side being a tine bit lop sided. I also notice that no loo seat I've ever had can be fitted in such a way that the rearmost seat feet touch the pot, the strain is all on the brackets. Not even bye removing the gaskets under the bracket hinges can fix this, and its all makes of seat, You would need to make the feet bigger at the back. Maybe I've just got a very out of specc toilet, but I'm blowed if I'm going to the trouble of fitting a new pan just so the seat looks straight and the feet hit the pan!
Brian