How can you compare worlds fastest man to the average of many different race horses? Bolt might be fast, but can he keep up that top speed for 50 miles? Were the horses doing 100 metre dashes? Was bolt carrying a horse or at least another man on his back? Was bolt weaking leather boots and assorted medieval gear? When they said clocked at finish line, did they measure speed at the instant the horses crossed the line, or did they measure it by averaging, like they did for bolt. That might be relevant too because the horses may be faster (last push) or slower (fatigue) at that moment.
By what justification do you extrapolate to declare that this is the correct ratio of speeds? Are we all suddenly the worlds best 100 metre sprinter, fighting against riders of the American quarter horse? Did you get the stats for the fastest quarter horse?