Ten pounds worth or Ten pound worth?

Hello, it's me again.

I would like to know why sometimes we say "ten pounds of petrol", although it's quite natural to say it, is it grammatically correct?

I know that if a noun is used to modify another noun after it, the first noun is normally singular, like "shoe shop" not "shoes shop"

But sometimes I heard that some say "arms control", "antiques dealer" with "s" at the end of the first nouns.

Could anyone tell me why?

Many thanks in advance.