I understood what the theory behind it had to be, but not having Synesthesia, I was left staring at a circle of seemingly random numbers.

Thank goodness for the forums... They provided me with an answer without doing a LOT of work on my own. Spoiler in the link, for those who like figuring things out on their own, don't click the link. You have been warned. (And sure, I would have guessed at this answer, as would have much of the rest of the 0-deck/io9 crowd, but knowing for certain is nice).

Subtext: "If you see two numbers but they're both the same and you have to squint to read them, you have synesthesia, colorblindness, diplopia, and myopia."