I encountered an odd and irritating thing. I have a 5 Bard / 5 Red Dragon Disciple with crafting feats, and maxed crafting skills (+ intelligence bonus).

So I made a Belt of Agility +2, which requires a caster level of 8. No problem. I'm wearing it now.

Then I went to add a +3 enhancement bonus to my shortbow, which requires a caster level of 9. It says "Your caster level is not high enough." If I up my Bard level to 9, it works!

So all I can see is that it's inconsistent. I get that Red Dragon Disciple isn't a spellcasting class, but the game seems to accept it as valid with the Belt, but not the weapon enhancement. What gives?

This is actually correct by pen and paper rules. The caster level of the item is only a binding prerequisite for weapons and armor; it's not a requirement for other classes of magic items. Yes, it is inconsistent and misleading.