writing differences that I noticed

[ololololo]

Apparently, you're referring to the traces of retouching.
On the f8r, the letters (towards the end) are slightly worn out and more "dull," and it seems that at some point, some of them were almost completely worn out, so one of the owners decided to fix them.

Well, you said the r-word, get ready to face the retouching/retracing inquisition 

I'm aware of only a handful of places in the manuscript where some retouching obviously took place, some examples in the image below. Most of them are consistent with running out of ink and overwriting the text immediately with fresh ink.

If you think you see obvious examples of retouching on f8r, let's have a closer look, could you show them on the images?

As I understand it, the author was referring to something like this:
   
The black arrows indicate the "bolder" letters. This could be a retouching, or the author could have underlined them to make them more visible. The orange arrow shows an example of a letter being erased, with the top line of the p almost invisible.