It should come as no surprise that hard drive platters have different thicknesses:
The thicker ones measure 1.25 mm, which is near enough to 50 mils to suggest they date back to the Good Old Days. The three thinner ones in the middle are 0.77 mm = 30 mil and could be slightly younger than dirt. There’s more where these came from and I expect more variation on the theme.
The beveled edges make the platters look thinner than they really are; they’re firmly clamped together with no space between them.