The numbers say that we have higher rates of "devotion" (attendance, adherence to rules, etc...) among our more educated/wealthier members than we do among our less educated members. It's a positive correlation. That's all.

You can assume reasons for what's makes that happen, but the data doesn't say one way or the other. It's possible that, as you suggest, our poor members get educated once they join. Or that we're a country club where educated people feel comfortable. My anecdotal experience supports the latter more than the former, but maybe yours is different. But the data just doesn't say one way or the other. It just says that we have higher rates of devotion among our educated members. And that's the opposite of the rest of the religious world.