The reason might be due to rank limitation. Nowadays DDR3 R-DIMM CPU can control max. 3 channels, 3 DIMMs per channel, 8 ranks per channel. If we installed Quad rank modules in each channel, then we can only install 2 modules (4ranks * 2 = 8ranks). If we installed 3 Quad rank modules in one channel, then it will not be working properly (4ranks * 3 = 12ranks, over 8ranks limitation). If we use Dual rank modules, we can full load the system with 3 modules per channel (2ranks * 3 = 6ranks, below 8ranks limitation).