I just read this bit of info on a web site.

PAA stamped marine plywood has consistent structural properties. For design purposes marine plywood can be considered to have a stress grade of F14. Marine plywood generally has thinner face veneers over thicker underlying cross veneers to give it similar strength and stiffness properties in both face directions

What do you think about this?. It does seem right that if you have 3 plys that the single one will have to be thicker than the other 2 so, as stated above, the plywood is as strong and bends the same in both directions.
The site I found this on is below.