it's all a matter of interpretation and how the algorithm for rendering is implemented.

for instance, look at a line weight at various levels of zoom in illustrator and you'll see a remarkable difference in the weight even just at different magnifications. this has gotten better in recent releases of ai but is still imperfect.

or try opening a pdf from illustrator in acrobat and you'll lose line weight almost completely (anti-aliasing goes away, nothing fractional, no matter what the zoom).

java and p5 have other versions of the algorithm that's used that probably aren't as "exact" as illustrator but are better than acrobat.