LOL, I don't think you will get an answer for this. But from various searches online, it should be relatively easy to infer that most people should clear Cat 2A/2B without issues.
Unless you do something wrong enough not to clear it.
security clearances are many levels - to determine if a person can handle/see certain levels of information.
Some information may seem trivial - but accumulated over time/different people can form useful intelligence that some countries/people are interested to use against Singapore.
As to how/why a person is granted a certain category, its classified - official SECRET. Nobody who really know will tell you how/why someone is a certain security clearance.
No matter what - all official matters to do with civil/govt service are on a need-to-know basis. It doesn't mean if a person is cleared a certain level, he/she can see everything....only if it pertains to the person's necessary work.