He made a minor mistake. In his high estimate, he used the high end of the range for de-duplication savings (20%) when he should have used the low end (5%). Not a big error (something like $4.5m/mo instead of $4m).
I don't think that's a mistake on his part. If you put a single large video or iso file on there that you downloaded, it's practically guaranteed that someone else has already uploaded it.
And one 700 MB file will really skew that percentage of deduplication savings.
