The challenge with measuring battery life, especially across users, is that there's so little consistency in how we use machines. My own concern isn't whether I get the same battery life as someone else but rather if my machine is getting the battery life it should get based on my usage. In other words, that it's not defective somehow (a serious concern with a machine that cost me over $3400 including taxes and Complete and minus my 10% military discount).
I'm guessing that those who are getting 14+ hours of battery life or whatever aren't actually hammering at the machine for hours straight. There's time spent away from the machine, the machine is put to sleep on occasion, and etc. Also, it's easy to point at Chrome and Slack, et. al., as problems, but if you need to use them then their impact needs to be factored in.
My own "7-8 hours" estimate is based on my literally working on the machine for that amount of time -- at most, I might take a 5-minute bathroom break occasionally (yes, that work habit is hell on my body). And that estimate also wasn't based on the machine being in battery saver mode nor limiting what apps I'm using to stretch battery life further. I'm sure if I tried I could stretch to the same kind of usage, but I'm naturally more concerned about how the machine holds up with my own usage patterns.