If you made the request the night before, then the hotel may have made an advanced booking for 530am, which would cost $5.20.

Though there aren't hordes of people fighting for taxis at that hour, it can still be hard to get off the street as the taxis often cluster at certain hotspots. Back when I lived in SG i could have difficulty calling for a cab at 6am in the morning, but if I walked out to the main road I can easily get one who'll have to drive back to my condo to get my luggage.

For hotels, they may not want to risk a guest missing their flight due to unforseen circumstances and so pre-booked a cab.

For an 8am flight on a public holiday, i'd have headed out 630am at the earliest!