Gordan Mah Ung at Macworld recently spent some time researching battery issues with the new MacBook Pros. He did comparative tests with different tasks and came to the conclusion that the problem is the MacBook Pro getting stuck on the discreet graphics card, which chews through battery much faster than the integrated graphics. Some of these issues were improved with the most recent macOS update but still your mileage may vary. I’m getting, fairly reliably, seven hours on my new 15″ MacBook Pro, which is less than Apple’s advertised 10.
I think with this trend toward hyper-tuning the hardware and software towards battery life, estimates are no longer as reliable as they used to be. For optimal battery life these days, you need to satisfy certain conditions (like not using the discreet graphics card) and a lot of users, like me, stray out of those perfect conditions often.