MTN Everyday gigs

I have a problem with MTN South Africa everyday gigs bundles is like every time you buy a bundle for a month the next month they increase the value of that bundle with a different bundle of a higher amount, when you call them they tell you it's no longer available as it is a promotion and if so why do they increase the bundles with higher one's when the last one they were offering was affordable to you I'm confused is like the more you buy the more they increase what is this?