I have no problem paying for the amount of energy I use over a month so if I provide my meter reading each month why not just pay for what I owe instead of estimating it it should be a bill not a savings account
Bulb make it clear on joining that you pay in advance, they take your estimated yearly usage and divide by 12 to arrive at a monthly payment so it is the same payment every month regardless of what you use.
This is how tariffs with a predefined monthly payment work. All supplies with this type of account work the same way. Many people prefer this type of tariff because they don’t know how to budget for variable bills.
If you want to pay each month just for what you use, you need a quarterly in-arrears tariff. Bulb do not offer one, so you’ll have to switch to a different supplier. Note that such tariffs are usually more expensive than the monthly direct debit tariffs.
To be honest I’m struggling to see why you didn’t realise this is how it worked before joining. If you don’t like the system, surely your research before switching suppliers would have told you this type of tariff wasn’t for you?