The percentage isn't the percent of it occurring but the area affected. So 51% of thunderstorms means 51% of that town/county/etc. will experience thunderstorms.
That's not how it works. Probability of precipitation means exactly what it sounds like: the probability that precipitation will fall in a specific forecast area.
Interesting. The way I explained it was how it was explained to me by a group of weather forecasters. For instance, here [0], is an explanation from a random weatherman that is in-line with how I was told.
the real question is what is our probability of Launch Commit Criteria Violation at T-0. i've seen them be no go at t-10 with a 10% chance of go and watched it lift off on time.
Reminds me of the line from The Naked Gun "Doctors give him a 50% chance of living, but there is only a 10% chance of that."