I actually have the same issue as you and I consider all previous answers either wrong or a blatant lack of knowledge.
In any circumstance, the available memory should not be affected if the internal GPU is disabled, and other than this, there seems to be no other resource that would reserve that amount of memory [MB not MHz]
Setting different values of memory reserved for onboard GPU seems to not affect the available memory [strange !]. If this doesn't affect, then something else does and somebody should give a better answer.
Also, setting of voltages and XMP profiles is faulty and barely functional, at best.
The BIOS/MB failes to properly read XMP values, and even though it is displaying 1600 in the calculated value [performance] it ends up. Manually setting the frequency to 1600MHz works but voltage doesn't, still keeps 1.5 and for the memory I am using I require 1.65.
I had another board, a Gigabyte with a Z87 chipset, which did not have those kinds of issues. [available memory and XMP]
I sincerely hope it's only a matter of BIOS, which can be solved, otherwise I am dissapointed in the product that Intel put out, compared to the quality they put out before.