It's just the rule for personal pronouns following prepositions taking the genitive. So mea, ta, noastră, voastră for 1st and 2nd person, but lui, ei, lor for 3rd person.
That's if it's following a feminine form, eg "deasupră". If it's following a masculine form, eg "în jurul", then you'd use meu, tău, nostru, vostru. Or lui, ei, lor again.