Convenient Timing

In D&D and other roleplaying worlds, there is an unspoken law of conservation of experience.

Player characters kill monsters and gain their experience in some sort of weird exchange, like a quickening from Highlander. The amount of experience in the universe is thus maintained. Balanced. As if on some form of cosmic scales. This balance is ruined when a PC dies and is not resurrected: the monster does not gain experience for the kill and the xp is not shared among the party. The xp is just lost. But nature will not tolerate this, as experience can neither be created nor destroyed. And so a new character suddenly gains that experience, levelling up out of the aether and appearing before the party.