"3/4" plywood" isn't 3/4" thick either.

Why plywood shrinks less than dimensional lumber

Plywood is made from thin wood veneers glued together in layers (called plies), rather than milled from a single solid piece of wood. It still goes through sanding, which removes a small amount of material from nominal thickness — but it doesn't go through the same drying-shrinkage process as solid dimensional lumber, since the veneers are already dried before they're glued up. That's the main reason the nominal-to-actual gap is smaller for sheet goods than it is for something like a 2x4.

Where this actually matters on the job

The thickness difference is small, but it adds up in a few common situations:

• Underlayment and flush transitions — stacking nominal vs actual thicknesses across a doorway transition can leave you slightly off from a perfectly flush finish.
• Dado and groove joints — if you cut a groove sized for "3/4 inch" plywood using a 3/4" router bit, the panel will be loose in the slot, since the panel itself runs thinner than the bit.
• Shelving and cabinet specs — manufacturer cut lists and cabinet hardware are often designed around actual sheet thickness, not the nominal label.

For anything where a tight, precise fit matters, the safest move is the same advice as with dimensional lumber: measure the actual sheet in front of you rather than trusting the label printed on it.