This all works only if everything is steady in life and it isn't. Today you work and save tomorrow you got a kid and your spendings take off and fly. Sometimes you want to celebrate something so you go to a bar and drink with friends which will kill your "latte-savings" for 2 weeks. Then you wake up and your tooth is killing you. you have to do a root canal and even with insurance it will dig into savings. Graph is too simplified. It will have spikes and drops - depends on each persons life and experiences...