Accuracy in attodollars for daily storage calculation

["Teresa L. Johnson" <tlj@ravenwebdev.com>]

Greetings:

I'm a new user of Tarsnap and have decided to use it as part of a backup solution
for both home and work.

After running a backup run, I was curious about the numbers and wanted to recreate
Tarsnap's charge calculations.  For one server, I correctly matched my numbers for 
total bandwidth calculations against Tarsnap's. No problem.

But my match against the generated daily storage amount was incorrect for the last 
9 significant digits. 

Total bytes stored: 721777623 bytes
Price charged: 0.006984944482595037 dollars

Using `bc -l` to recreate the calculation, I get the following:

721777623.0 * 0.0000000003 / 31.0
.006984944738709677

The comparison:

Tarsnap: 0.006984944482595037
Mine:    0.006984944738709677
                    ^^^^^^^^^

At first, I thought my system was returning the last nine digits as trash, but as a control,
I can calculate the repeating decimal 1/3 and pi correctly with no corruption in accuracy
up to zepto-units.

1.0/3.0
.333333333333333333333
4*a(1)
3.141592653589793238460

So, is the definition of 'daily' not 1/31 (for the month of August)?

Out of curiosity, how is the daily storage in attodollars per byte-month calculated in our accounts?

Thanks,
-tlj