According to The hooligan :

Someone mentioned higher voltage being pumped through the lines. Does
this make sense to you who are not electrically challenged? How about
more Hz?

No neither would explain it. Voltage or hz changes big enough to
have this big an effect on your billing would almost certainly
have caused damage to some of your or your neighbor's equipment,
and would probably have been visible during the time.

Actually less hz means more current to inductive devices like
motors, and more hz means less current. hz changes won't do
_anything_ to pure resistive devices.

With many devices, when you raise the voltage, the current
doesn't increase at the same rate, and in some cases even declines.
In a pure resistive device, a 10% increase in voltage comes with
a 10% increase in current - 21% in watts. However, many devices
in a home don't behave that way. Even incandescents significantly
change resistance as the voltage goes up (filament gets hotter). In
other words, V/I isn't a constant in incandescent bulbs or resistive
heat strips.

I'm more thinking of a processing error in their billing cycle. If
you know what the before/after readings were (directly from the
meter) compare that to the bill. It's remotely possible that when
presented with a big gap in the data flow, the billing software
gets confused - eg: a negative increment instead of a positive.
Who knows, perhaps the meter electronics forgot something while
they were unpowered _that_ long.

You seem to be lucky that your utility is also interested in
understanding what's happening. Keep working it - they will
want to figure it out.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.