A person who lives in Singapore calls himself or herself 'Singapore citizen'. A Malaysian citizen lives in Malaysia. What is the reason for this difference?
It is logical if the nationality of a Singaporean is Citizen of the State of Singapore. However, a Singaporean calls himself or herself a Singapore citizen. I think it should be more logical if he or she is a Singaporean citizen, just as a citiizen of Malaysia is a Malaysian.
In short, what I don't understand is why it should
Isn't it easier to say, Singapore citizen, than citizen of Singapore? Other than that - ask the Singaporeans. What they wish to refer to themselves as, and others to do also, is of their choosing.
The normal practice in Singapore is to use "Singaporean" only as a noun (to refer to citizens of Singapore) and "Singapore" as an adjective. Hence "Singapore citizen", "Singapore government", "Singapore passport", "Singapore food" etc.
The CIA World Factbook recognizes this distinction: