I misread your initial question, so I deleted my answer as to not sound pedantic.

I believe the simplified expression is on the left because probabilities, for some reason, are read right to left.

Because this is Bayes' theorem, I believe it's interpreted as "The probability of the event happening is equal to the conditions relating to that event."

Jeez, I hope that was the answer you're looking for.

(makes a mental note to expand into statistics and probability classes)