Ounces per square yard measures how much cotton is in the cloth. It does not measure how that cotton is arranged, and arrangement is most of the answer.

Duck is double-filled

Two weft yarns run together against one warp yarn.

The result packs tighter and resists abrasion far better at the same weight.

So the comparison inverts

A 10 oz duck routinely outperforms a 12 oz plain weave.

It is also lighter to post, which shows up on a large order.

Yarn is the second variable

Combed cotton has the short fibres removed, so it is stronger and smoother.

Two cloths at the same ounce weight and weave can still differ here.

What to ask for

Ounce weight, weave and yarn, in writing, on every line.

A supplier who will only give you the ounces is telling you something.