It's one of many attempts to produce a scale whose notes are in tune in more than one key. It's produced by a series of arithmetical manoeuvres using whole numbers - the smaller the ratio between two numbers the more consonant they are.

C E G has a ratio of 4 5 6, so does F A C and G B D The keys of C F G work well with this scale, but other keys are progressively more dissonant.

The logic is that it's pretty close to Pythagoras without using ratios like 256/243. It's a compromise, as are all tuning systems.

Stu