Because most of the atmospheric neutrinos are produced via pion decay chain:

$\pi^- \rightarrow \mu^- + \overline{\nu}_\mu$,

$\mu^- \rightarrow e^- + \nu_\mu + \overline{\nu}_e.$

As the end result of pion decay, two muon neutrinos and one electron neutrino are produced (1:2 ratio). See picture below for intuitive view.

Most of the atmospheric neutrinos are produced via pion decay chain:

$\pi^- \rightarrow \mu^- + \overline{\nu}_\mu$,

$\mu^- \rightarrow e^- + \nu_\mu + \overline{\nu}_e.$

As the end result of pion decay, two muon neutrinos and one electron neutrino are produced (1:2 ratio). See picture below for intuitive view. This is for the muon neutrinos from above the detector.

Regarding the neutrinos arriving to the detector below: they are created in atmosphere other side of the world and travel through Earth thousands of kilometers, and the neutrino oscillation effects are important. Half of the muon neutrinos oscillate (or transform) to electron neutrinos in-flight. The characteristic length of oscillation of these neutrinos is order of hundreds of kilometers, so the above-arriving neutrinos (which are created approx. 15 km above the detector) have had no time to transform to other neutrinos.