I’d hazard a guess and say that the tiers are determined by the value of the handset you select.
BT-M will want to make a profit, so would take into account the cost of the handset and the wireless subscription on a 24 month basis and add a % to that in line with market standards.
I doubt they’d officially disclose the calculation as it could be deemed as commercially sensitive but you could reverse engineer it by looking at the handsets page and working it out.
