As far as I can tell it’s always been down to the cards going in. Bronze+Bronze gives 1/1 to stats per level, then fusions with a silver or gold get increased to 2/2 and 3/3 respectively. While this means Bronze combos make weaker fusions on bronze, silver and gold cards, it also means gold makes stronger ones! Try comparing a combo that involves a gold card and makes a silver or bronze card- such as combining Time+Tree and Water+Tree, even at the same level you will see the gold card makes a much stronger one.
I don’t particularly mind this feature; it just makes gold and silver cards more useful than bronze.