I think Dizzy means Anniversary event for the 500% exp, and not Christmas event. I believe these oddities started happening after the update earlier this year to remove server boosts stacking for cookie quests (I can't remember if it was all boosts or only select ones that we decided shouldn't stack, maybe @Matt remembers). Although it seems like some of the server boosts are stacking?
Here's an example from anniversary event. I assume the +50% Drop Rate comes from the event as this was in the last week of the event. Unfortunately Rare Enemy Rate isn't shown so you can't see how that's behaving.
EDIT: Actually, maybe these boosts are correct, except for the event boosts stacking. Can't remember how the quest boosts were changed..
I was under the impression it was changed to be a flat 200% rare drop / rare enemy / exp while ignoring other boosts (they stopped the stacking because of people hoarding and stacking with huge event boosts afaik) but yeh, the numbers have seen all over the place since those changes. RER on the /partyinfo would also be dank asf