I'm not an expert in any way. But my understanding is that they did/do test on a lot of different axes. But what they found was that people tend to have stron

I'm not an expert in any way. But my understanding is that they did/do test on a lot of different axes. But what they found was that people tend to have strong correlations between those scores. As in: if you are good at one, you are good at others. Not exactly as good, of course. But a strong correlation. So that's how they arrived at using a "single number".