It's not a "slightly" better rating, they are 500% safer riding rear facing. And it applies to all types of crashes, not just rear end
https://www.car-safety.org/rearface.html
ETA I don't know about there but all seats have to pass the same safety tests so all are completely safe to use as long as you're using them correctly and they are installed right. The best thing to look for is something that fits your car and your child best.
It's all about how you view the statistics. And a safe front facing seat is better than an unsafe rear facing seat. And at the moment, the rear facing seats meet only the basic safety standards. The best front facing ones will out perform them by miles.
It is the same here, all seats have to pass a Eurpoean Standard safety test, but this doesn't necessarily mean they are all as safe as each other. Some do better, some do worse.
I'll give you an example. Which? are a consumer organisation who do independent testing on all sorts of things from kettles to cars. I've just had a look at a video they produced of two front facing car seats, which are on sale in the UK, which have passed the same European Safety rating, in a simulated crash. The difference in how the two perform in the tests is remarkable. The poorest performing seats would certainly cause far more injury than the best ones. The reason they pass the Eurpoean tests is that they are only tested to the equivalent of a 30mph crash, and they don't include a side impact simulation, whereas which tests to 40mph and checks for SIP to. I'd love to be able to post this video but Which? is a subscription service and I'm not sure the link would work.
This is no different to any other product you buy built to meet safety specifications. Some will meet the minimum requirements, some will exceed them. Which? are currently campaigning to have the European Safety standards revised.