Question:
When I allocate RAM to a Netscaler instance on SDX, the Netscaler reports considerably less memory than is allocated.  For instance, if I allocate 16gb, the Netscaler instance shows only 12gb allocated.

Answer:
This is an expected behavior. When NetScaler is started and is initializing the physical memory; of the available memory left over after the FreeBSD kernel allocates the memory that it requires, the Netscaler packet engines will give back some additional memory back to FreeBSD.  This additional memory is  either 2.5gb or 25% of the remaining free memory, whichever is smaller.