They key to this question is that while the value of the stock decreases and increases by the same amount, it doesn't decrease and increase by the same percent. When the stock first decreases, the amount of changes is part of a larger whole. If the stock were to increase to its former value, that same amount of change would be a larger percent of a smaller whole.
Pick a number for the original value of the stock, such as $100. (Since it's easy to take percents of 100, it's usually best to choose 100.) The 20% decrease represents $20, so the stock decreases to a value of $80. Now in order for the stock to reach the value of $100 again, there must be a $20 increase. What percent of $80 is $20? It's $20/$80 x 100%, or 1/4 x 100%, or 25%.