Nonplussed

I rang Plusnet for my PAC code so I could swap back to Giffgaff.
The girl offered me a better deal that isn’t listed on their website: 9 quid for 5 gig 2500 mins and unlimited texts. I said ok. She said they’d send me a 4G SIM card.
When it arrived I rang to register new SIM.
The chap said oh yes I can see you’ve taken the 10 deal. I said I was told 9. He said ok I’ll do it for that.
Later I received an email confirming that I’d taken out a 5 gig deal with unlimited texts and minutes for 6 quid a month. Which is less than what I was paying before.
I’m not complaining but I’m wondering what sort of system they have in place for determining tariffs.