As explained well by Nate Silver's "Signal and the Noise", when the weather forecast model says a 50% chance of rain, that doesn't mean it "might" rain, that means that in 50.001293% of millions of simulations precipitation above some threshold developed in the spot in question.

A news station can't get away with saying 50% though. The public assumes that's a cop-out answer. As such they will say 60% even if the real number is 50%. They will never say 40%. Better to seem over-prepared then rain on you when it wasn't supposed to.

Even if we had the perfect computer, at the end of the day it's got to get to your average person through a local news weather reporter.