What's light usage in your definition? With nothing but Safari open and a number of tabs of browsing going on (15? 20 maybe? I could do far worse, but trying to be conservative for battery use), 6.5 hours is a good day to me. I could only see getting 10 hours in a single tab (or nearly so) time, but that's hard for me to deal with these days with all the compute power enabling never waiting for a tab, just flip to the next one. Only extension is a content blocker API adblock, which should be a benefit to battery life.
That's what I think the problem with web browsing tests on most websites and Apples own testing is, it's a simple dumb script that loads a page, emulates reading for a number of seconds, and then repeats. Most users who are more than the most casual will change tabs the second they're waiting on one.
Edit: Nice coincidence actually, Gruber just posted some battery life tests, and his method was exactly what I described as not replicating a user but still had under 10 hours
https://daringfireball.net/2017/05/safari_vs_chrome_on_the_mac