I'm not certain, but it seems that the collectable you get from feeding your pet may be related to the tier that the food came from. In other words, feed with food from level 20 drops, you'll get level 20-ish collectable.
Luckily I had been saving up the plant food for ages before RoK came out. Once I got my pet, I had several stacks of food to feed it with. I got quite a few low level collectables amongst that lot. Now the stacks have run out, I'm mainly getting food drops from level 60's and above. The collectables I'm getting all seem to be for higher level collections (like the hollow wasp sting I got last night & put on broker for over 7pp!)
Just a theory so far...