Something is wrong here... I am not reaching 20Mh/s so where is my 9.4% or 1.68Mh/s? (17.76Mh/s vs 16.08MH/s)

I have 2 RIGS and always the same Hash Rate per GPU = 2.2Mh/s (X15)

My specs:
Rig 1: 4x MSI Radeon HD 7990 (underclocked @ 950MHz, undervolted @ 1.1
Rig 2: 3x MSI Radeon HD 7990 (underclocked @ 950MHz, undervolted @ 1.1
Windows 8.1 Update 1 and the latest drivers from Windows Udpate 13.251.9001.1001 (2014/07/04)

Test 1 - Maxminer.net or Suprnova.cc @ Rig 1 (8GPUs)

• sgminer 4.2.2 (build 2014-07-20): 1.990Mh/s / GPU
• sgminer 4.2.1 (build 2014-06-25): 1.840Mh/s / GPU

Result: The latest version of sgminer 4.2.2 is 7.5% faster than sgminer 4.2.1 build (2014-06-25) but still 9.4% slower than original speed 2.2MH/s.

Test 2 - Maxminer.net or Suprnova.cc @ Rig 2 (6GPUs)

• sgminer 4.2.2 (build 2014-07-20): 2.010Mh/s / GPU
• sgminer 4.2.1 (build 2014-06-25): 2.220Mh/s / GPU

Result: The latest version of sgminer 4.2.2 is 9.4% slower than sgminer 4.2.1 build (2014-06-25).