You are right.
I'm sorry for the issue with the memory size.
This is the real board booting log shows:
DRAM: Initializing....using SPD
Detected UDIMM 9JSF25672AZ-2G1K1
Detected UDIMM 9JSF25672AZ-2G1K1
Detected UDIMM 9JSF25672AZ-2G1K1
4 GiB left unmapped
6 GiB (DDR3, 64-bit, CL=13, ECC on)
DDR Controller Interleaving Mode: 3-way 4KB
The memory surely is pre-installed.
The issue was caused by the RDB board documentation issue (QorIQ T4240 Reference Design Board Quick Start, Rev. 2) where the log is:
DRAM: Initializing....using SPD
Detected UDIMM 18JSF51272AZ-1G9K1
Detected UDIMM 18JSF51272AZ-1G9K1
Detected UDIMM 18JSF51272AZ-1G9K1
10 GiB left unmapped
12 GiB (DDR3, 64-bit, CL=13, ECC on)
DDR Controller Interleaving Mode: 3-way 4KB
DDR Chip-Select Interleaving Mode: CS0+CS1