Example taken and adapted from Wikipedia: in a stationary population where 51% of people die at the age of 5 and 49% dies at the age of 70, the life expectancy at birth (defined in the sentence immediately before the [2]) is 36 years 10 months, whereas the median lifespan is 5 years. Life expectancy is the “same as mean, or average lifespan” in stationary populations no matter how skewed the distribution of death ages is -- the reason why it isn't in real populations is that mortality rates change with time.