Is it possible that over the years the number of nuclear devices per warhead has increased? 1, 4, 8, etc. per warhead, thereby increasing your capability while claiming a reduction?
What you are thinking of is (or used to be, called a MIRV, a mulitiple independently-targeted re-entry vehicle). There's still only one atom bomb per warhead, but 6 warheads per mirv. Think of the MIRV like a cylinder on a revolver with 6 warheads per MIRV like the bullets in it. The warhead has the minimum amount of hardware necessary to make it blow up, while the MIRV has the computers and rocket engines on it. So…
Reagan's so-called "trust, but verify" policy.
I find it difficult to accept that any party to those agreements would actually reduce anything without having equivalent plans B, C, and D.
I am not a weapons expert so thank you for your insight.