As I understood it, the basis for their system was that each customer's account contains the actual current balance (remaining value) at any point (any given day). If you charge $10/year for a service that's 2.739726027 cents per day. You can't have a system like that and round to pennies without some serious error at the end of a year.

I'm not sure I understand the value of a system like that though. In the systems I've used and built, most customers see their service as an annual thing that renews on a particular date every year, for which they pay an annual fee. Doing it as a daily fee simplifies the special cases like converting between plans but makes it harder to deal with "your annual plan always renews on this date".