EDIT: Who says the truth?
Reason is very simple. You have installed the express version of catalyst driver, which even if you not selected AMD overdrive to OC, controls the clocking ability & it completely denies & don't allow to hash more, even though you are lowering mem clock & it is shown in Afterburner & also in Overdrive.
AMD overdrive controls the mem clock in background which you can't see.
Simple solution, Uninstall all the catalyst driver using driver sweeper & then reinstall using latest or any catalyst driver with CUSTOM install & install only these 3, ATI display driver, ATI catalyst install manager, APP 2.4.
Check this, http://forum.bitcoin.org/index.php?topic=6458.msg168021#msg168021
Lowering mem clock reduces temp up to 10C & also increases hash rate.
If you have 6870 card use these, mem clk = (core clk/3) + 14.
If you have 5870 card use these, mem clk = core clk/3.
This gives the MAXIMUM hash with MINIMUM temperature.