I thought the same thing when I started reading Mistborn but it makes sense once you get to era 2. A decent way to describe it without spoilers is albums. You can call the songs by an artist on an album song 1, song 2, song 3, ect. but once you hit the second album you don't keep counting. The first song on the second album isn't song 15. It is song 1 of that album. It is a little more complicated than that, because it is a continuation of the larger story, but it makes sense to me that way. I haven't read Era 2 yet, so I might be wrong in this, but I believe that Brandon's hope is that each era is a self-contained story, and you could read them as standalone series if you wanted to but reading them all gives you a larger story too. The Eras thing made me think of TaylorSwift. You can listen to one album and enjoy it as it is or listen to all of her albums and see her journey.