Why is my otherwise healthy laptop's battery giving significantly below par performance? And weird anomalies in power drain!

I have the XPS 13 9360. It's a i7 kaby lake with 16 gig ram. And UHD.

I've seen numerous review sites on the web saying that the new XPS (yes, they refer to the UHD version) has about 8.5 hours battery life during a wi-fi Internet surfing test. Some sites even claim similar battery life when playing a 1080p movie on loop. They mention at the screen brightness is low, about 50%

While wi-fi surfing with Battery Saver on, and my brightness at the lowest level, ie 10%, I get a battery consumption of almost exactly 10% per half hour.

Five hours of battery life with everything set to the bare minimum, and only wi-fi surfing.

No music on, nothing. If there are any background apps running, they are few and far between, I haven't really loaded much software onto my Dell aside from drivers and Dell Support apps. I mainly use it for Office Productivity and web surfing and watching movies. 

I just need to add: the websites I've been on are only slightly media rich, i.e. maybe a GIF banner ad, but certainly not YouTube or Facebook. I guess Facebook is what you'd call a "live webpage". I do have gmail open, however. Tabs open: 10. Just about all media-low.

Surely this is ridiculously short battery life considering what most people are saying average is i.e. 8.5 - and I have checked diagnostics. According to the Dell Battery app, my battery is in excellent condition and is operating normally. The amount of battery life I'm losing out on is waaay more than half of what I am getting! 

BUT here is the strange part. I turned Battery Saver off, and upped the screen brightness to 30%. There's absolutely NO change in the rate of battery drain. Then I tried maximum brightness with Battery Saver OFF. And the drain rate was only slightly faster than the 10% brightness setting - at 12% power drain per half hour. I would have thought there'd be a significant difference

My question is: do I have a defective battery, albeit only minimally OR is this a strange software issue, I have the second latest BIOS (I'm installing the latest tonight, but it shouldn't be that, all the reviews for UHD screens with i7s 16r ram came out with older BIOS versions than I'm using now).

I mean, really, 5 hours battery life while surfing at minimum brightness and Battery Saver on. This has to be a hardware issue, right? I can't believe I'm losing out on 3.5 hours!