This is not unlike the weight classes in boxing, where the categories below 200lbs are tightly defined but weight differences beyond the Heavyweight class can be widely disparate. A 210lb fighter for example will be in the same weight class as a 320lb boxer, but a
160lb fighter cannot fight a 154lb pugilist. In terms of SD speed rating, as long as a memory card can sustain a minimum write speed of 10MB/s, it is given a Class 10 rating.