At this point it's basically like car models years. They rev them annually and make a few improvements but they're actually going longer between major redesigns. If they didn't bump the numbers every year people walking into stores would be worried about paying lots of money for an old phone that will soon be replaced by a much newer model.
Liquid Glass has been refined significantly in the 27 OS's and you have more control over the degree of transparency. Overall stability and performance are better as well.
