Why are cosmic ray muons decaying more slowly than predicted?

Muons are unstable elementary particles. They are heavier than electrons and neutrinos but lighter than all other matter particles. They decay via the weak interaction. A muon decays most commonly to an electron, an electron antineutrino, and a muon neutrino:

The mean lifetime, τ = 1/Γ, of the muon is (2.1969811±0.0000022 ) µs. That means that every 2.19698 µs the population of muons is reduced by a factor e=2.71828.
An experiment compared the population of cosmic-ray-produced muons at the top of a mountain, whose height is 2 km, to that observed at sea level. Those muons were traveling at 0.95c, where c is the speed of light, so they arrive to the sea level t=7 µs later. At the top of the mountain the measured population was No=563 muons per hour. Therefore, according to the decay law, the expected population of muons at the sea level should be:
$$ N=N_0 e^{-\frac{t}{\tau}}=23$$
muons. Nevertheless, 413 muons where measured, so the muon sample at the sea level was only moderately reduced! The muons were decaying about 10 times slower!
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