Mach Kernel vs. XNU
The documents is called/ mach_kernel for historical factors ; XNU is, without a doubt a crossbreed of Mach and also a BSD layer (neglect Wikipeida, there is http://www.amazon.com/Mac-OS-iOS-Internals-Apples/dp/1118057651 which appears extra clear-cut).
In reality, this kernel isn't the specific photo which obtains crammed - there is a pre connected kernel cache (with all the essential kexts) in/ System/Library/Caches/ com.apple.kext.caches/ Startup.
Based on what the Wikipedia page on XNU claims, XNU is based upon Mach. Given that OS X began making use of the Mach kernel and also currently makes use of a by-product of it, they possibly simply maintained the kernel is documents name the very same so they really did not need to fine-tune firmware throughout upgrades or with new equipment launches.