Salary range is probably on the lower end of what they’re willing to pay for the right candidate. No one wants to pay more than they have to for employees who are as yet unproven.
As for accepting a first offer, there’s nothing wrong with doing it. But, its a completely personal decision based on your relationship with your future employer. If it’s a place you want to work, and you’re happy with the offer, why not take it? Asking for extra after a particularly friendly interview might leave a bad taste in their mouth. If you feel like they’ve given you a low but acceptable offer, there’s no harm in asking for more. It’s totally circumstantial, though.