In USA they do not use the same Octane system as the UK, in the USA the pump fuel is rated in PON (pump octane number), in the UK and Europe we use RON (research octane number).

PON is a value that comes from the RON and MON (motor octane number) value of the fuel and is a more acurate measurement of fuel rating than our RON system. To get PON you take the average of the RON and MON value of the fuel. (PON=(RON+MON)/2)

91PON is aproximately 95RON
93PON is aproximately 98RON