Was the paint added later? And is the artist also the scribe?

[Jorge_Stolfi]

I think the observation that the same pen might have been used for text and drawing is worth investigating

The problem is that quill pens wear out relatively quickly -- especially when writing on vellum, which was often coated with chalk or other minerals to smooth out the surface.

Thus, after writing a bunch of text (guessing: a couple of pages), the tip of the pen had to be trimmed and re-sharpened.  And then it would write thin strokes at first, then gradually wider as it wore down again.

Moreover, the Scribe may have re-shaped the quill's tip when switching between text (where a broader stroke works better) and drawings (where the thinner the trace, the better) .  Or used different quills for each task.

And, finally, the width of the stroke depends totally on the pressure that is applied with the pen onto the vellum.  Thus, even on the text, the tails, plumes, loops, and ligatures are thinner than the glyph bodies because they are traced with reduced pressure.

All the best, --stolfi