Not to confuse a confusing subject, there are different way of calculating octane as well. Research Method (R) is the theoritcal value that the octane could be; M Method (i can't remember what the 'M' stands for, but it is the actual real world (measured?) value and most race fuel is rated on this scale) and the average at the fuel pump - there is a sticker on the side of the pump for the octane method: R + M / 2
If in a discussion of octane values/compression ratios be sure to get which octane rating method is being used.