The chamfered pressing boards are designed to provide a normal force to the paper of the board which promotes a uniform paper surface. Many people will place a piece of silicone release paper between this board and the from and back cover when pressing. others will simply put the board behind the first interior page of the book.
The chamfer (technically it is a fillet) is designed to allow you to do this better than a regular board because it doesn’t leave a line along the spine when pressing. This board and technique may help with spine tics, but that is not its primary function.
Due to the placement of the staples, the board can rarely snug up to the spine perfectly and there will be air gaps. for this reason, unevenness of the board is acceptable. Fabricating a perfectly even edge is possible but that method nearly triples the costs of the boards themselves with no measurable benefit.