Answer:
Let's compare two scenarios (to get a feel, try it with the engine above):
The engine shows, chances that the S&P 500 will
(1) rise by +5% or more, over 20 trading days (about 30 calendar days) is about 11%
(2) drop by -5% or more, over 20 trading days (about 30 calendar days) is about 8%
Clearly, there is a slightly better chance the S&P 500 will rise by a certain amount, than fall.
Though the same trend bears out for many other values of rise/drop and over other time periods - it isn't always the case.
For example, the engine shows, chances that the S&P 500 will
(1) rise by +10% or more, over 20 trading days (about 30 calendar days) is about 1.2%
(2) drop by -10% or more, over 20 trading days (about 30 calendar days) is about 1.5%
In this case, the chances of a 10% drop are slightly greater than of a 10% rise