billing question: why do we have a huge rounding error cost?

I was checking our aws billing and was surprised to see that the line for "This line contains rounding error due to the consolidated billing and hourly line item calculation processes" was not the usual tiny number of around +/- $0.05 but was $698.361159! This can't be right.

I got a report for Aug01-Aug07 and the rounding error total there was -0.003. For all of July it was -0.005, for June it was 0.05, for May it was 0.02, etc so it's usually tiny, as one might expect. But when I did a report for Aug01-Aug13 suddenly it had shot up to $698! It's definitely in the right column and it's being added to the AWS pre-tax total (it's included in the tax calculation), but if I just manually add up all the pre-tax lines in the report they don't include this $698 so I can't see where it's coming from - so I think it's something that AWS is doing and not a formatting error on our part.

I'm guessing it's either something that we did in the past week (though I don't think we've done anything new or unusual in that period) or some kind of AWS accounting glitch - does anyone know what could cause this and how to stop it? Or even how to find the individual rounding errors used to calculate this total?