Aetna's (NYSE:AET) ObamaCare exchange statistics should clear up any doubt as to why the Obama Administration has been tight-lipped about enrollment since celebrating 8 million sign-ups in mid-April. Reality, evidence suggests, could require quite a come-down from those lofty claims. The nation's third-largest health insurer had 720,000 people sign up for exchange coverage as of May 20, a spokesman confirmed to IBD. At the end of June, it had fewer than 600,000 paying customers. Aetna expects that to fall to "just over 500,000" by the end of the year. That would leave Aetna's paid enrollment down as much as 30%...