Deflection is the key, Danny, and Cassandra's point... <going technical here all.. skip the rest if you don't want to read technical>
when you hang a weight on a flat plate, to exert its lifting force it bends downward slightly. It's the compression force on the top and the tensile force on the bottom that provides the "lift". Just how much it deflects depends upon many things such as thickness, distance to support and properties of the material used. As long as the weight doesn't exceed the "elastic deformation" range and move into the "plastic deformation" range, it will spring back when the weight is removed.
Thus technically, if you want it to be perfectly flat with a weight on it there should be some amount of crown on it without weight such that after the load is applied it becomes flat. In practice, for a rigid plate that's not excessively large and not too heavy of a weight, that amount is insignificant and can be ignored.
For an easily visualized example of this, consider a long overloaded bookshelf. It sags in the middle. If it were designed with a slight crown in the middle and the ends were constrained (held) from moving outward, it could hold a lot more weight. This principal is what makes arched entries so much stronger than square ones and why a lot of the Greek stonework (with arched brick construction) are still standing today.
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