So many people get confused by this concept but once you break it down its pretty simple.
If you are attacking a Player's City, your troops that drop to Zero HP default to Dead and will either need revived (only during SvS) or are gone forever.
If you are being hit by someone or battling someone in a building that is not a City, then your troops that drop to Zero HP default to Wounded and will fill up your Hospital. Once your Hospital is at Capacity; any more troops that reach Zero HP, regardless of the situation, will be Dead.
Why do I get Wounded when Attacking people and Dead when Defending?
If you look closely you will find several things (Techs, Generals, Ect) that modify a Troops Status POST battle.
So you will most likely end up with a mixture of Dead, Wounded, and Surviving Troops in every battle!