It's all about the tailwinds. And a good leadout and/or pack.
The segment closest to my home is .4 mile long with an average uphill grade of 1%. The top 10 riders all did it at over 27 mph. (I'm #8 at 27.6 mph and I'm 57 years old.) The KOM did it at 33.2 mph!
How are we so fast? It's a combination of (1) huge tailwind, (2) a fast pack working together leading someone out, and (3) for the KOM, the fact that he averaged about 700 watts for 54 seconds.
Not many "mature" KOMs are accomplished without a massive tailwind and/or the assistance of a pack of cyclists.