Does this vary more depending on the plywood grade, maybe? I have some shelving I made from AB birch, and it looks pretty close to .75. Maybe 1/64 scant. (Although I don't have calipers handy and just used a tape measure, so I'm not being super precise.)

Lots of plywood, especially birch, is metric-sized. You might have 18mm plywood on your hands, which would be about 1/32 under 3/4 inches.

The plywood I often buy at a big box home store is labeled as 23/32" (0.71875" or 1/32" less than 3/4") and it is actually that thickness. I don't recall seeing any 3/4" labeled plywood though.