In the beginning of the season 5 finally Jacob says, "Everything only happens once, everything before is just progress" I believe that this means that every thing or everyone makes progress everytime time repeats itself. An example is in the past time loop Jack may have only gotten the bomb down to the well but not able to detonate it. This left the bomb there for desmond to explode and negate the engery. This time however, Jack was able to explode the bomb, with the help of the other losties. This could be the progress Jacob was talking about
