On the surface, this certainly seems like a cut-and-dry case of technology being weird, or companies doing weird things for weird reasons. I'll explain a bit how processors work, the differences, and hopefully at some point I answer your question!
BLUF: Tablets can suffer a bigger hit on battery life to allow for better and faster processors, where phones employ an energy-saving processor combination to be more efficient.
Processors work in a myriad of ways, all dependent on the model, year made, company, and purpose. When you have things like dual, quad, or octa-core processors, they have the opportunity to process instructions and data depending on the current use of the device.
If you're just browsing your home screen, you don't need a lot of processing power, so the device will employ the least energy-intensive processor core it can to do the task. When you start doing things like playing games, or watching/interacting with 3-D images/videos/etc, it becomes more processor-intensive, so likely the device will employ all of its processors and, in turn, drain a bit of the battery.
Phones with octa-core processors will commonly be only truly dual-quad-core processors, where they have two sets of quad-core processors, one of which runs constantly, handling all of the basic tasks, but then turns the second set on when more intensive things are required. This is (usually) quite energy efficient, where one processor can be very good at doing basic things at a small energy cost, instead of having multiple processors (or, just one really energy-dependent) processor running all of the time.
Where tablets can spend a bit more on internal hardware and battery space, they can allow for the dual or quad-core processors that may be a bit better in terms of the ability to do complex tasks at fast speeds, but at a bigger cost to the battery, since the device can suffer that hit.
If you want to read the article that I drew some of my information from, click this link.