The 3.5 inch figure represents the floppy, remember that? :)
A "3.5in HDD" is a hard disk that fits into the bay originally designed to hold a drive for 3.5" floppy disks.
But no, looked it up, 3.5" have platter sizes of 3.74" apparently, 5.25" had 5.12" platters. Which makes sense when you look at a hdd with the top off.
(3.5 * 69.9 / 102 = 2.3985)