Okay, back to the original topic (in a desparate hope to not turn this into yet another discussion of whether the listed caster levels are prerequisites).
As to why a ring would have a caster level lower than 12th, there's not much reason. There's no price break for making rings with caster level lower than you can manage, unlike with some other types of magic items. There is, however, at least one reason I can think of.
When two or more characters cooperate in the creation of a magic item, the person with the Forge Ring feat may not be the item's creator for purposes of setting the caster level. That does make a strange default assumption for the caster level of a type of magic item, but it is one reason why you might do this.