So partypoker is rating players now, regarding the bonuses

Since different players got different bonuses this month at partypoker, presumably partypoker is now rating players and giving us different bonuses depending on how much we bet or what level we play at or whatever else.

Any theories as to exactly how they're doing this? I'm wondering if it's now better to leave some money in one's partypoker account, or whether completely cashing out would be fine, etc.