writing differences that I noticed

[ololololo]

The only way that would logically be possible is if it is actually a key to a cypher of some kind, or a way to identify the lines themselves.

Or they were very difficult to see, which is why they were outlined
The reason could also be a malfunction of the pen - it could pour ink "quickly", which made the first strokes very bold, while the rest were duller. At least, that's what happened with my iron pen.