Anyone who uses OpenCart store with large filter list knows that default filter logic might be a bit strange and a unexpected. For example, if you have filter groups Color and Size, and customer selects Color->Red, and Size->5, OpenCart shows all products that have red color, and all products that have size 5 at the same time. Instead of showing all red pruducts that are size 5.
In other words, default OpenCart does not combine filters with each other. So you may need an extension that can change that behaviour.