Same applies to Pumpkins, Disk of returning and Easter Egg.
But if you think about it, the half wine jug is actually the rarest item among all other types of rares. Party hats and Christmas crackers were dropped to people and they were intended to be picked up and worn. Same thing applies to Easter Egg, H'ween masks and Pumpkins. Although the Disk of returning was not a "gift" from Jagex, players who wanted to visit their friends in the black hole (place where all banned people are sent to), had to purchase it.
But who kept a half wine jug on their bank that was worth less than 5 coins in their bank?
Nobody. Imagine that one day, Jagex gives a success rate of 100% to cook shrimps. How many people would have in their bank "burnt shrimps"? Not that many. Scenario would be a little different and more complicated with the half wine jug because only experienced players would be able to know how to use or make wine jugs... So banking "half wine jug" would be really not that common, thus is the reason it is considered the rarest item among all other types of rares.
Party hats and crackers, especially after getting duplicated before, shouldn't be considered the rarest just because they are the most expensive. It is just because you are actually able to wear them and show it off that their prices are that high..