I'm not sure if this is related to this:
https://roadmap.thestorygraph.com/de/bugs/posts/pages-goal-percentage-rounds-up-to-100-

the percentage of my "pages" reading goal shows up as "100%" on my profile page, but as "99%" on my challenges page.

I tested it on pc and on my phone, which has two screens (1 square & 1 narrow), it's the same on all of those screen sizes.