Some systems will vary slightly in how they calculate a "Gigabyte", depending on whether it's dividing by a factor of 1000 or 1024 per byte. There are actually separate notations for these different types of file measurements, as can be seen in this chart:
Indexed IO uses the GB notation, with a 1000 byte factor. If you need to look at your sizes in a different notation, our downloadable billing report has a breakdown of processing and hosting sizes by byte. For further reading, check out this article.