Think about it like your a manager at that company. Not a person chasing miles for AA status

They (giftcards.com management sales team) probably have their own personal sales quotas they are chasing. They might offer a 3x commission when sales are slow or before another big bonus which will bring in sales. They were a ZERO miles for a while also this year! They have historical sales data so they know something like a credit card promo or payal promo brings in huge sales when it shows up so they reduce the miles payout. They can get the same sales numbers with less commission they have to pay to the portal.

Those promos on paypal or your credit card. The merchants pay a commission to those companies to get on there, its not a FREE thing your credit just offers randomly. Its all business decisions of when and how much they offer. In this case giftcards.com doesn't want to pay a double commission for your sale so they reduced it, makes total sense