RSDS is part of the debug information section. The structure looks like (using the construct library's syntax):

  CV_RSDS_HEADER = Struct("CV_RSDS",
      Const(Bytes("Signature", 4), "RSDS"),
      GUID("GUID"),
      ULInt32("Age"),
      CString("Filename"),
  )

So it's a GUID, and one property of GUIDs is that they're "Globally unique", hence regenerated each time. Mystery solved.

Edit: Here's a source [1] so you don't have to take my word for it.

[1] http://www.godevtool.com/Other/pdb.htm