Because our phone companies are 99.97% marketing. They're constantly trying to market deals and segment their market. So today it just so happens to be 120GB cheap. Next week it might be extortionately expensive. When 5G first came out it was ridiculously expensive to get decent data caps, but I would assume that they now have excess capacity and so they're trying to compete with some other competitor at that price point.
> using the least expensive plan with a (minimum) data allowance of 500 MB over (a minimum of) 30 days
Is just rubbish.
It's just impossible to price things like this, take EE's pricing per GB for a 1 month sim:
* 1GB - £14 - £14 per GB
* 120GB - £20 - £0.16p per GB
* 10GB - £20 - £2 per GB
* 200GB - £23 - $0.115p per GB
* Unlimited - £35
The total price of the cheapest contract is £6 cheaper than the next step up, which is 100 times cheaper per GB.