Omega, why 120m?

While on the hunt for my most recent purchase, an Omega Seamaster Polaris, I noticed that several Omegas from the 80s and 90s have 120 meter water resistance. Maybe it's just me, but I thought it was odd. One sees 30, 50, 100, 150, 200, and 300 meters often. But 120 meters? It doesn't appear to be something Omega (or anyone?) does anymore. Is there any practical reason or interesting story to explain it?