How the bill estimator works
In the bills and deposits area, you can estimate a bill or deposit amount based on the last X number of instances. You can choose a number of instances from 1 to 12. See Article 286
These instances must be entered through the bills and deposits area. The estimation is based on only these transactions and will not use existing transactions in your account register (unless they have been entered through this same bill and deposit entry).
If you delete any of the bills from the account register which are being used to calculate the estimate, Money will remove it from the bill history used to calculate the estimates. When doing so, the bill and deposit entry recalculates based on the information it has left.
So, if you removed all of the bills in the account register that the entry in the bills summary page is using, you will have an estimate of zero. When setting up a new bill with no history, you will also see a value of zero.