HDD sizes are set by manufacturers according to the IDEMA "LBA Count for Disk Drives Standard"[0] formula:

  LBA_count = 97696368 + 1953504 * (Reported_Capacity_in_GBytes - 50)

So, for 120 GB HDD:

  (97696368 + 1953504 * 70) * 512 / 1024^3 = 111.79 GiB
  (97696368 + 1953504 * 70) * 512 / 1000^3 = 120.03 GB

For 200 GB HDD:

  (97696368 + 1953504 * 150) * 512 / 1024^3 = 186.31 GiB
  (97696368 + 1953504 * 150) * 512 / 1000^3 = 200.05 GB

For 500 GB HDD:

  (97696368 + 1953504 * 450) * 512 / 1024^3 = 465.76 GiB
  (97696368 + 1953504 * 450) * 512 / 1000^3 = 500.11 GB

Doesn't look like powers of 2 at all. Are you sure about your sizes?

[0] http://www.idema.org/wp-content/downloads/2169.pdf