I know you know this, but I'll indulge you anyways. ANSI standards for bicycle helmets are not as high as for equestrian helmets. It's not the speed that matters but the height. You're falling from maybe 5 feet on a bike and from closer to 10 on a horse.
Yeah - I 'spose. But bike helmets ought to be designed for a likely tangle with concrete or a car, and for much higher speeds, so I'd think the height difference in the design should even out ;~D