It hasn't been totally confirmed but just because you got an exception item despite having both Catalogs on does not mean this isn't correct. Basically, the game runs a triple dice roll: It rolls once for a Rare item (Which has a 1% default appearance, up to 5.5% with Conn. Catalog + 5-star rating), if that fails it rolls for a Common item (which has varying percentages, usually higher than 20 but with a max of 37.5%), and if that too fails it will roll for a Shroud. Shrouds are independent of the formula.
Basically, one enemy will only ever drop one item. The catalogs conflict because you can only ever take precedence with a Rare or Common item. Since an enemy will never drop more than one of either, if you want Traps you should only have a Conn. Catalog on, and if you want Ingots, you should have a Collector's on.
HOWEVER, there are exceptions. As I said, it's all a dice roll. I've gotten Ingots with only a Conn. Catalog on, however, my chances of getting a Trap are overall higher because I only have that catalog equipped.
Until someone totally disproves this, I'm sticking with it. People who consistently get Traps or get it on the first turtle just got a lucky dice roll. Fucking RNG.
Edit: The max a Fang or Vanille weapon trick is pretty good for getting Traps if you are unsuccessful through farming. However, it requires having at least one Trap. So either make the Gil or get the drop. You'll still need about 1.25 million Gil to max even the first weapon, though.