Why do some depression sufferers experience insomnia while at the same time others experience hypersomnia?

Depression, Sleep. Depression affects many functions of the body including sleep. At age 30,At age 30, stressors of relationships, marriage, parenting, work and finances can cause Depression or Depression with Anxiety. Depression can cause Hypersomnia. Depression with Anxiety can cause Insomnia. Unresolved emotional conflicts cause brooding, difficulty to sleep, or waking in the middle of night. or Hypersomnia.
"Atypical" depressio. Good question! we don't know specifically why "atypical" depression exists - only that it does. Not only is there hypersomnia, but also increased appetite, feeling of leaden paralysis, and rejection sensitivity. It accounts for about 40% of people presenting with depression, and is 4 times more common in female patients. http://www.ncbi.nlm.nih.gov/pmc/articles/pmc2990566/.
Circadian rhythyms. Sleep is basically controlled by a clock in our brain. Depression is believed to cause problems with the clock. This can lead to certain sleep disorders where there is not enough sleep and not restful sleep or to excessive sleep.