Point is that you simply cannot use efficiently same chip on tablets, laptops and desktop. Because usage scenarios are so different.
Even Apple isn't trying to use same chip on phones and all above. That pretty much proves my point.
Why it has to be Same CPU? Having same CPU instruction set is more than enough to allow applications work across devices. Also if power requirements are different, chips are working differently and will not offer same performance/experience. That means Apple should have used different chip to get optimum experience.
The “same chip architecture” is more or less the same as “the same chip.” I’d consider the Intel Core Family to be the same chip despite an i5 being different from an i7.
The usage scenarios for various form factors are converging rather aggressively. For basic computing tasks at the home and office, a tablet or even a smartphone can replace a PC in most areas. Applications are increasingly cloud-based web apps like Office 365.