While quite possibly you do end up with a lower battery life in your tests than the one advertised, you need to take into account the following:
Test conditions
Many factors will determine the outcome when testing the battery life, like signal strength, iPhone settings, screen brightness, the actual websites being loaded, background processes and much more. FYI, apple discloses it's test conditions, you can read about them here. If you truly want to compare, you should as best as possible mimic the test conditions Apple used.
Test accuracy
While I have no problem taking you for your word your battery drops 1 percent exactly every 5 minutes while browsing, that doesn't necessarily mean you end up with 8 hours and 20 minutes of battery life. There is quite a large margin for error when you only sample such a small period, and the only real way of checking Apple's claims would be by actually trying to surf the web for 10 consecutive hours.
Concluding, while Apple's test conditions are semi-optimal for their testing results, they have a better reputation for test accuracy than some other manufacturers. They do test the iPhone with almost factory default settings, but specifically for browsing the actual web sites being loaded can still make a noticeable difference in the test results.
From the info you provided, your battery seems perfectly fine to me and within the margins of difference one would expect.
Best Answer
I had this same problem for the first after updating my iPhone 6s to iOS 11. I wasn't using Safari as I use Chrome but it would consume up to 50% of my battery and I was having to charge more frequently then before.
The solution was to disable iCloud syncing for Safari:
Settings → Apple ID → iCloud and turn off Safari.
My phone is now behaving 'normally'.