Occasionally my pets earn 1 experience point each, or 1 point and, say, 3 points, even though the battle wasn't easy (one battle that earned a combined 2 experience points was in Faerina's Shrine against three Myconid creatures, for example) and even in the easy parts of the map when they were lower levels they usually earned a minimum of 100 points.

I was wondering why this happens?

Is there some item or something that's reducing the amount of experience points that my pets should be getting, or does it have something to do with levels/difficulty?

It's really frustrating when I fight a battle that reduces my pets' health significantly, only to get two or three experience points at the end.

This happens when the area you're fighting in is 10 levels lower than the pet you're using to fight.

Such as using lvl34 pets to fight in a lvl10 dungeon.

Just gotta fight in an area that fits the range of your pets within 10 levels.