Odd Language #65: Unusual Singular/Plural Use

Puzzle #68: Binders has an unusual sentence.

The issue: The sentence “You bought three red, two blue, and one green binder.” is unusual. Should it be “binder” or “binders”? Apparently, the rule is that whether to use singular or plural depends on the singularity/plurality of the last item in the list. It still looks odd to me.