Good question. Bills are issued according to a pre-established yearly billing schedule. The billing date, and consequently the due date, may vary from month to month, particularly so they will not fall on a weekend. For instance, if the due date is a Sunday, it is moved to the following Monday. This means that intervals between bills may be a bit longer of a bit shorter than 60 days.
We are currently studying different billing options. One such option is fixed-date billing, for which we are assessing technical feasibility and development costs.