As a rule of thumb, we can think about most data being in a certain range, given the variability of the data.  There are very few ten feet tall people, for example.  But the assumptions of the normal distribution apply sometimes (heights) but not always.  This is especially true in financial markets and gave rise to many books, including the Black Swan (the book, not the movie).  Today, oil prices moved down, and when compared to the recent movements in the market, the change was over five standard deviations.  Although it rarely happens, it happens all the time, because parts of the world are not normal.