So you only pay the current bill debt, not the total debt, and the total debt will generally be more than the current bill debt.
The calculation fee is not calculated according to the consumption in May and June as you said. According to the consumption before May 12 and the consumption after May 12. The date when the node is 12 is not from 1 to 3 1 in the usual sense.
Of course, there is another possibility that you have cashed in or paid in installments, because both cases will generate handling fees and interest. If there are no these two situations, it means that you spent money after the date of 12 mentioned above, and it will be included in next month's bill.
All hands up, hope to adopt.