Simple reason. In Singapore, u bring to Apple Singapore, they check, and replace.
In Malaysia, you bring to Apple authorized service centre, they check, then take it and send it to singapore by end of the week. Then Singapore received it early next week, verify and replace then send back to Malaysia service centre. Once reached, they call and you go collect.
I think the time frame is about 2 weeks?
Anyone who buy from apple online store Malaysia will received their phone from Apple singapore. Thus it's also a Singapore set. When you tell the service centre it's a Singapore set, they might assume you received your set from Apple singapore, since a lot of Malaysia purchased their phone from Apple online store Malaysia.
As mentioned previously, iPhone do not have international warranty. I have been asking Apple service centre the same question again n again and they told me the same answer..
I told them already that's Singapore set, I bought in September, they say can claim as as well it's come out from APPLE ONLINE STORE