You are correct Bob. The companies save a bit of money by undersizing the plywood. Newbie, the common sets of plywood bits are for 1/4", 1/2" and 3/4" plywood and are undersized to match. This does not mean that all plywood is undersized, and the only way to be sure is to measure before you make a cut. Plywood also comes in 3/8" and 5/8" and this too can be standard or undersized. If you are not going to be building a lot of items requiring dado's or are tight on the money for tools remember you can buy the undersized 1/4" bit and make multiple passes until you get to the required width for your dado.
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