It seems like the base bet is not based solely on VIP level, but also recent action.
A plat 2 that wagered $6k over the past 6 months is likely going to have a lower base amount than a plat 2 that has been wagering 60k a month for the past 6 months - and if both players wager 0 next month, I would expect the second player to still receive the higher base amount.
You claimed a couple months ago that you don't play at Stake any more. If that's still the case, and you got $12, then $12 is the base amount, which, imo is generous.
It is clear now that base amount for monthly bonus is not solely based on our VIP rank but also based on our wagering activity.
Of course it is understandable if players get less amount than other although they are at the same VIP rank.
Casino is a business, so there must be a good calculation as well as policy for the promotion to make it worth for the casino.
Above user is no longer actively playing (as per his own claim) but he still receive the bonuses although not as much as before, yes it is still generous act from Stake to give him the bonuses.