No, they have a "battery budget". It the CPU power draw goes down that means the budget goes up and you can spend it on other things, like a nicer display or some other feature.

When you say "up to 10 hours" most people will think "oh nice that's an entire day" and be fine with it. It's what they're used to.

Turning that into 12 hours might be possible but are the tradeoffs worth it? Will enough people buy the device because of the +2 hour battery life? Can you market that effectively? Or will putting in a nicer fancy display cause more people to buy it?

We'll never get significant battery life improvements because of this, sadly.