Recommended D/D changed twice in a few weeks

Billed on 21st March, my D/D is £63 a month, after bill generated my recommended payment reduces to c£60 a month, I decide to leave it at £63. Log in today and my recommended has gone to £63.47!
How can Bulb change the amount when nothing has actually changed, no readings give as not required until 18th May,
I understand it can change after a bill, but not in-between, you have no extra data to analysis to recommend a change

2 posts were merged into an existing topic: Paying first bill in advance?!