AT&T bills me a different amount every single month: I am not using a new service or feature, their massive billing software just can’t keep it consistent. Of course ACH is appealing to them for this - I can’t complain about the amount debited then.
AT&T's billing system (Telegence) is so convoluted, even their own staff can't tell you what your month to month charges will be, it's why they only give you an estimate when you sign up for a new line.