The Pro 3 models have a special built "Haswell" series of CPUs. These are not the ones that you put in a desktop or laptop even though, they would work if they built a motherboard for them.
For the Pro 3 series, these particular CPUs are flatter and wider. If you want to see a picture of the i5 that is in the current shipped model, go to http://www.ifixit.com.
The "Haswell" series is a whole wide range of CPUs and chances are, you never seen or heard of the others. These particular CPUs are built just for the Pro 3 models.
The circuitry within them are exactly the same as the regular desktop and laptop "stacked" CPUs that you normally see and those are designed to fit the LGA 1150 "Haswell" socket. The desktop CPU version was used in the Pro and Pro 2. So, YES, it is still the 4300U but the chassis of the CPU is redesigned for these tablets.
They did that to flatten them and allow greater cooling for a thinner computer case.
Did you know that they made a Pentium and Celeron versions of the "Haswell" CPUs?
Broadwell will be the next Intel generation coming out.
You keep reffering to the 4300U as a desktop processor, used in Surface Pro 2. That series are ultra low voltage laptop processors. They can, and are, used in desktops as well. But they are primarily laptop processors.. They don't hold a candle to normal voltage real desktop processors... The Surface Pro's are far too thin etc to be able to take on desktop processors.