why do bonus terms change after you claim them sometimes

This has tripped me up a few times, mostly on reload offers and special promos tied to table games. I’ll read the terms before I claim and everything seems fine, but then right after, the requirements have changed. Sometimes it’s higher rollover or they quietly add excluded games. Once, I had a cashback bonus go from 10x wagering to 30x after claiming.

I don’t get why the terms can just shift like that. Is this normal across most sites, or should I be doing screenshots and contacting support every time? Wondering if anyone else has spotted differences between what’s advertised and what actually shows up after you click accept?