I've seen 54" elsewhere. Our sizes follow the guidelines produced by ISPA, International Sleep Products Association, which Sealy, Serta, Simmons, and Tempurpedic follow. So, at least 80% of all mattresses are produced to this size. Typically, the problems arise from the bed manufacturers not following these guidelines, but it is not a common occurrence. More likely, the size is reported incorrectly by the retailer or reseller.
Why are Spindle's mattresses a 1/2" short in the length?
Unfortunately, there is a lot of incorrect information out there giving wrong dimensions and conflicting information. For example, Mattress Firm, the largest mattress retailer in the US, lists a Serta mattress at 60" x 80", but Serta itself lists it as 60" x79.5".
Keep in mind that usually the bed frame is an inch larger allowing for the mattress to fit inside. Also, this provides some space for the sheets to be tucked in.