What does it tell you when you see that your site visitors have increased there time on the site? Is it that they are lost or that they are looking for products and haven't found what they want just yet!
I think when a visitor gets lost on a site he leaves and doesn't spend more time. But when visitors do spend time they are interested!
increased average time is a good thing. Keep in mind that the biggest variable that influences average visit time is the number of people who exit immediately. Lower that figure and you end up with more people who stick around once they land. This will show up in better stats, as it has with you. Congrats. You've made a change that seems to be working.
Or maybe they are looking at another website while keeping your browser opened. Maybe a phone call came in while they were shopping. Maybe an email came in and they had to respond. Maybe that email was a notif to a post on a forum and they went to reply on the forum and got stuck there answering other questions.