Noun Adjectives: Singular or Plural?

Hello,

In English, there is a general rule that says "noun adjectives" are usually used in the singular form. For example, we say "a shoe store" not "a shoes store". However, this rule says "usually" which means we will find noun adjectives used in the plural form. For example, I found this in an English writing textbook: "identifying words in a common words list".

Why do we sometimes not follow the rule of keeping adjective nouns singular? Would changing the noun adjectives to singular or plural make any meaning difference?