In real life, there's not much of a difference. But they are not symmetrical. See, negligence is defined in terms of carelessness, not the other way around. Negligence is basically a lack of reasonable care. Gross negligence is basically really bad negligence. So I'd say "extremely careless" is a way for a guy who happens to be an experienced lawyer to hint at "gross negligence" without saying it, because if he'd accused her office of gross negligence explicitly, it would have been very difficult for him to explain his final recommendation.
In other words, I believe Comey concerning his reasons for not recommending charges, and I think he was personally unhappy about it, because he thought Hillary was guilty, but knew that they couldn't make a case in court, so he carefully crafted his statement to be as scathing as possible, while still remaining consistent with his official recommendation.