On mine, the build plate is visibly crowned and the straight edge will rock slightly when checking. This action as well as the possibility of a less than perfect zero is part of the reason for the longer cure times on the first few layers. The bottom layer's complete function is to attach and to level before to start the part build and with sufficient bottom layer cure times it is pretty forgiving of error. It is able to cure the resin across a slightly larger gap when things aren't perfect and establishes a perfectly flat, uniform surface before to start the part build. The over cure will cure across a larger gap as needed but will also cure a larger area around the perimeter from the additional light bleeding around the edges in X and Y causing a dimensional error in those planes only, tolerable because it doesn't matter as it will not be part of the finished piece.
The only other thing that I see that might be different in what I'm doing from many, but is also exactly as the directions suggest, is I'm using 99% alcohol and making sure I have really cleaned the plate, taking a final wipe with a clean towel and fresh alcohol. I'm not saying this IS the problem but that maybe it helps. I can't say for sure because I haven't tried anything different BUT I also haven't had any problems with attachment, crowned plate and all, and not redoing the zero procedure since the initial setup.
