The more I think about it, the more it sounds like sizable chunk of data being unaccounted for in the early lifetime of the drive. If there were 100GB unaccounted for in the beginning, its impact as a percentage would get smaller and smaller as writes increased, basically asymptotic behavior. I haven't done any math to see what size that would be, but it might match up nicely with the attempts of getting the right amount of static data on the drive (or even the 39GB itself).