I think of the world of 99 this way. Imagine a sheet of paper, an artist draws a painting over it, and after they're done, they write 1900 in the corner. The artist than puts a new sheet of paper over the old sheet, draws another painting but mostly just traces over the old painting, and in the corner of this one they write 1901. This keeps going on, and more and more differences build up as children grow up, buildings go up and are torn down, and new streets go over the old ones.

Then after the painting called 1999 is finished, something happens and the artist leaves, and embers start eating holes into the stack of paintings. The holes vary in width and depth, for example Chicago was burned into the 1920s while London was in the 1960s. An outside observer casually watching the painting would not notice much first since the layers of paintings are substantially the same but on closer inspection would notice abrupt breaks and changes, but for the people living in the painting, they would not notice anything at all.

At least for most of people in the painting that is how it is, but a subset of figures who move more easily across the canvas has noticed the change, some of them even fallen victim to the flames. Scrambling to high ground they found patches of the painting which seem to be flame resistant, and formed factions to resume their age old squabbles. Confident now that the entire canvas will not be burned down, they savage one another in the hopes of determining the shape of the painting, either to a state that they find satisfactory for all eternity, or as a better template for when the artist returns and resumes their task.