There are at least two ways of measuring octane, because the US "CLC" system is actually an average of those two... next time you're at the pump look for the label that says something like:
93 Octane (R+M/2)
they're showing the averaging formula on the pump
The [R]esearch and [M]otor octane measurement systems are defined in ASTM standards in the US (see below). The Research octane number (the one used in at least some portions of Europe) is typically 4 points higher than the US CLC (average) number.
ASTM D2699, "Standard Test Method for Knock Characteristics of Motor Fuels by the Research Method", 1991 Edition.
ASTM D2700, "Standard Test Method for Knock Characteristics of Motor and Aviation Fuels by the Motor Method", 1991 Edition.