In fact advance booking does cost S$8 so you were charged correctly as per this service. The misunderstanding was between you and bellman here I think. Looks like yours were current booking considering the time frame and not advanced one. I would be surprised too, there are plenty of taxis nearby which comes up to Hyatt taxi stand, so in any case there wouldn't be much waiting even without booking.

If it would be peak you would see S$7.05 upon entering a cab.

Usually when I was leaving Brix there a plenty of taxis waiting at GH taxi stand with no suspicious surcharges.