Gamasutra - Lost Dave Arneson Interview
They don't explain the context as far as I can see but Gamasutra has a "lost" interview with Dave Arneson. Apparently written in 2004, it hasn't previously been published:
How did you decide on probabilities and percentage chances of the various things you could do? How did you translate real world probabilities into dice rolls?
Well, I could tell you I had it all planned out, but that wouldn't be true. And I could tell you I faked it all, and that wouldn't be true either. We adapted.
We started out using the Chainmail combat system. They had a fantasy supplement for Chainmail. I think we used that for two games. We quickly discovered it didn't work for what we were doing since they were mass-combat rules, not individual rules.
We were doing role-playing and they weren't role-playing. We started off our monster list, and I think Chainmail had only seven or eight monsters. So we quickly came up with twenty or thirty.
We tried setting them up in a matrix, but that didn't work because it was quickly taking up an entire wall. So, I adopted a combat system I used for Civil War Ironclads because they had armor class, hit points, all that stuff.
And we did that for the monsters, we assigned values to them: giants are big, orcs are little. We tried to make the creature's power similar to what its size was.
We tried to give each monster a special power that wasn't overwhelming, which was harder than I thought. It's easy to come up with incredibly powerful abilities, not so simple to make small ones.