How does Google Analytics calculate "Probability of Outperforming Original"? Here's an example content experiment in Google Analytics. Can anyone explain how to calculate that there's a 98.7% "Probability of Outperforming the Original"?

 A: The percentage is calculated using Monte Carlo simulation, so reproducing the calculation here would not be easy. See https://support.google.com/analytics/answer/2844870?hl=en
A: It is not any basic or easy calculation to put here some equitation. Calculation of Google Experiments is based on the "problem" of Multi-armed Bandit.
This is a concept that describes any situation in which you want to conduct an experiment in such a way that you maximize your reward
Full description is available on Google Documentation.
Experiments based on multi-armed bandits are typically much more efficient than "classical" A-B experiments based on statistical-hypothesis testing. They’re just as statistically valid, and in many circumstances they can produce answers far more quickly.
They’re more efficient because they move traffic towards winning variations gradually, instead of forcing you to wait for a "final answer" at the end of an experiment.
They’re faster because samples that would have gone to obviously inferior variations can be assigned to potential winners. The extra data collected on the high-performing variations can help separate the "good" arms from the "best" ones more quickly.
Calculation example is here. 
I hope it helps you to understand a bit more how Google is calculating the winner.
