There are 6 golden keys in the garden and 6 golden jugs, but only 5 of the jugs can be opened. As long as you got the jug which gives you enough experience points to level up (and it sounds like you did), don't worry about the remaining one.
Imp world - Bee/wasp garden - map showing jugs, run around with the alt key depressed to find the keys, if you do not have them all
