Can you check how they search for it. They might search it worded different. Maybe check page rank is what they use
One thing to take note is that Yahoo Singapore is a small market since it caters to the Singapore market. Most people using Yahoo are usually the less tech-inclined crowd. The ones who probably search the term are normally using Google Singapore. I haven't used Yahoo Singapore for several years already.
This is the fault of the keyword selection. You should try to select a keyword that attracts a large number of visitors. In your case the keyword would not be attracting a large number of users, so that is why even though you rank high you are not getting traffic.