How posting decides who goes where
MOE posts students based on three things: their PSLE score within their Posting Group, the order of their school choices, and the vacancies in those schools. A student with a better PSLE score is considered first for their preferred schools.
When two or more students with the same PSLE score are competing for a school's last places, tie-breakers apply in this order: citizenship (Singapore Citizens, then Permanent Residents, then international students), then how high each student ranked that school, then computerised balloting.
That is why meeting a school's cut-off point does not guarantee a place: tie-breakers can still decide the last few vacancies.