Hi Th3Drizzle,
There are no exact sizing standards in the industry for mattresses or components and they can vary slightly from manufacturer to manufacturer depending on the dimensions of the materials in the mattress that they order and the thickness of the cover that goes around the foam layers (the foam layers are always smaller than the dimensions of the mattress itself).
You can see some examples in post #3 here …
A Serta “standard” king size is 76" x 79.5" +/- 1/2"
A Sealy “standard” king size is 76" x 80" +/- 1/2" to 1"
A Simmons “standard” king size is 76" x 79.5" +/- 1/2"
A Brooklyn Bedding “standard” king size is 75" x 79" +/- 1" - 2"
The final dimensions of a mattress are all “side effects” of the dimensions of the materials and components and any variance in the materials and components that they order from the suppliers that they choose to use in their mattresses and the dimensions they all add up to when the mattress is finished. More than this type of generic information though you would need to ask them directly.
Phoenix