It’s mostly because of the suffix -ius /jus/, added to nouns to form adjectives. Both the nominative plural and genitive singular of that suffix are -iī /ji:/, so if you’re talking about multiple of that thing or something belonging to that thing, it gets the -iī.
It gets worse in this word though. Classical pronunciation was likely /pom.pej.ji:/, both the first and second “i” were long, or like you had three “i” in a row. And this mess wasn’t even originated in Latin, but a Southern relative called Oscan.
It’s mostly because of the suffix -ius /jus/, added to nouns to form adjectives. Both the nominative plural and genitive singular of that suffix are -iī /ji:/, so if you’re talking about multiple of that thing or something belonging to that thing, it gets the -iī.
Here’s a totally serious video representing the true pronunciation of the suffix and totally not some internet meme. (Yup, it’s like that “yee”.)
It gets worse in this word though. Classical pronunciation was likely /pom.pej.ji:/, both the first and second “i” were long, or like you had three “i” in a row. And this mess wasn’t even originated in Latin, but a Southern relative called Oscan.