In the US we use either 1st floor and Ground floor to refer to the same floor. The second and higher floors are consistently named though, except for those buildings that skip the 13th floor.
Singapore is even more bonkers because they have eastern and western superstitions to accommodate, plus it's a really densely-built island so tall buildings are extremely common.
Not always, nothing like the US and inconsistency, I work in the northeast US on a college campus our buildings have G-1-2-3....even the newer buildings follow it.
When your country is made of tiny countries (states) with comparable sizes and populations to European countries there are always going to be exceptions.