Was It Ever Explained How They Keep Coming Back?

I know it's a silly, dumb question for what kind it show it is but I just thought there could have been a small, funny inside joke (BEFORE the Shampoo thing since Carl has died before using it)..even some episodes jumping ahead in time...although they could be explained as "What ifs"

I mean seeing the "Greatest Story Ever Told" after the last episode I like how Shake says "It's the first day on eternity because I've just found out I'm *beep* immortal" :D

Oh are we just going to take it as they've been using the same Shampoo for the entire show or without the shampoo they actually are immortal being crazy creations by some mad scientist