Our group has been studying paper case bindings. These come in a remarkable array of styles and were popular in the 18th Century. They are very close in structure to limp vellum bindings which date back to the 14th Century. They are fast to make and depending on the paper you use for the cover they can be a cheap but very durable binding.
We found a great array of samples from our collections to study. What I am realizing as we study these historic structures is that binders of all centuries seem to make it up as they go along. There are the canonical exemplars, the forms that have survived and were popular in their day, but the details show us that every binder did things a little differently. There is no one way to make a paper case, in fact there are several. You can lace the supports in or not, you can adhere the paste downs or not, you can adhere the turn ins or make yapps. Or not.
Likely this is due to the availability of materials, the popular methods of the day and who taught you. Judging from my own work, I also suspect there are so many variants because you make mistakes and need to fix them. Along the way you discover a better or quicker way to do things then adopt those “fixes” and pass them on. I like knowing that I’m just one in a long line of binders that never do things quite the same way twice.