Hi MarkM,
You can see the Simmons “standard” sizing by clicking on the "Standard Sizes—Do Beautyrest mattresses come in standard sizes? here (second from the bottom).
Their cal king standard sizing would be 83" long +/- 1/2"
If you have measured it correctly (and not just by the size of the gap between the mattress and your headboard) and your mattress is less than 82.5" long then based on their standard specifications your mattress would be out of spec.
There is more about mattresses that are out of spec in post #3 here and the posts it links to.
You are certainly not the only one that has run into this issue.
Phoenix