This is the correct answer. Most maids in SG live with their employers, and the employer has to feed them. Maids are typically from developing nations like the Philippines, Indonesia, Myanmar etc.

600 SGD goes a long way in those countries.

SG also doesn't seem that expensive; granted, I was only there for four days, but things like public transport and food was quite cheap. It's just the housing that's really expensive from what I heard.