I can't tell you what they are made of exactly, because there is NO tag in the shorts. I bought 4 pair of these shorts for my husband and he does like them. They are light weight and they don't shrink (so you may be correct on the percent of polyester/spandex). I like how they fit. They don't seem to get baggy throughout the day and the price was right!
I have a pair and they are 75% cotton 25% nylon. I'm trying to find more of them. They are really comfortable and I like the deep pockets.