The short of it - CR tested with higher standards that are currently required by the US Federal Government. However, these standards are what are required of car manufacturers. If you take a quick look, the standards overall are appalling for determining "safety":
Cars and car seats can’t be sold unless they can withstand a 30-mph frontal crash. But most cars are also tested in a 35-mph frontal crash and in a 38-mph side crash. Car seats aren’t.
When we (CR) crash-tested infant car seats at the higher speeds vehicles routinely withstand, most failed disastrously. The car seats twisted violently or flew off their bases, in one case hurling a test dummy 30 feet across the lab.
Consider the accidents you've seen, been in or heard about. How often was it at speeds lower than 30 MPH and just a direct head-on collision?
I found many articles and blog commentaries attacking CR for causing undue stress and concern among parents for using higher than federal standards in their tests, and among the experts for not revealing their methodology. Sure, you can hardly blame manufacturers for conforming with the existing standards. They have to manufacturer to some common goal. But, I don't think CR should get beat up over their tests, either.
I think the anger should be directed at updating these feeble federal standards.
Infant seats sold in England and the 24 other countries of the European Union must meet safety standards that include a 31-mph frontal crash. Many are also tested in a 40-mph frontal crash and a 31-mph side collision. The results are the basis of widely publicized safety ratings for cars. The ratings include an assessment of how a car seat performs in a specific car. Automakers are required to make those seats available for purchase.
Some FAQs about LATCH and car seats from babycenter.