Why am i a PR and my nationality is a malaysian?

i was born in negri sembilan (malaysia) and was 3 months old my father a british army then.. brought me here (singapore) in 1958, which was then still a malaysian state . when singapore seperated from malaysia we choosed to stay . i study and work here and never leave the country since the first day i entered singapore. my question is why am i holding a SPR ic and my nationality a malaysian when i was liberated here