Sometimes, when characters die, they come back to life thanks to the magical properties of 1 UP mushrooms.
If a character is somehow ressurected or reincarnated in a manner wherein a 1 UP mushroom is not utilized, then the character was simply not dead, despite the fact that they died.
Confused? Don't worry, we all are.
Rumor has it that pills are manufactured from the raw materials of 1 UP mushrooms (If this is true, it looks like I picked one hell of a day to quit amphetamines).