[enki]

because an attacker controlling less than 50% of the computational resources can be lucky and find the first solution before anyone else.

he can even be lucky twice or thrice. but to be lucky 12 times against everyone else with less than 50% of the resources, is statistically improbable.

2 hours is the result of those two values (12 being statistically relevant, and 10 minutes a chosen parameter of problem difficulty), not an input variable.