Well done! I'm sure they were very impressed.
I always offered people lower salaries in the first offer because it tells me how much they want the job, whether they can think on their feet, stand up for themselves and negotiate a better deal. There's no reason not to lowball if you consider it a stepping stone to a mutual agreement. If they don't ask for more, then from a management perspective, they are probably only worth the low offer because they have yet to learn that they need to have a decent regard for themselves and confidence in their abilities if they are to succeed. I realise that sounds harsh and that there are people who just need a job and it doesn't matter what the salary is, but I have to employ someone who will stand their ground too, and not get walked over in business. I want the sort of people on my team who can turn situations around to their favour because that's what I hope they'll do for me when they're working for me. So good for you!