I don't work for DELL, so I don't know why they would do that.
But here is what I think.... They probably do this so that they can have a "universal" GIG size as a standard replacement across multiple drive manufactures. Make sense?
As an example. If they were to take bids for 100,000 drives from IBM, Seagate, Maxtor, etc. they could then say we want all 36GIG drives to be seen as XX,XXX MB in size, that way they could issue "replacements" as other vendors, but rest assured that the replacement was the same size as what you had.