It is a little more complex than that.
Bulb requires you to pay for your energy usage monthly in advance. That monthly payment is set at an estimated Annual Usage plus two months, minus any credit then divided by 12 to get an average monthly payment.
every time you submit meter readings Bulb recalculates this estimate and provides you with a recommended payment and suggest you change that. The calculation takes into account how much credit have have on your account. However, this calculation has been at much discussion lately on the forum and in my personal opinion that calculation is flawed.
Typically bulb provides you with all the data to why they think you should increase payment. There a many people on here that keep track of their energy usage and make their own predictions, but the general rule of thumb is, as long as you are are able to pay your monthly bill in advance there is no issue.
you can look at this discussion to get a better idea how Bulb calculates the recommendation http://community.bulb.co.uk/discussion/comment/22311/#Comment_22311.