I am not quite sure why some sites are getting such different results even though they are using the same core offset. I don't think it is their power settings since most site turn it up to 135% when they start overclocking. Nvidia told reviewers that the GTX 690 should be able to reach a 1.2GHz clock speed and nearly every site managed to get at least that far. Out of all the sites, I think [H]ard|OCP is the only one that mentioned that they made sure that both GPUs on the GTX 690 were running at the same speed:
"..if you are gaming, and want to stabilize GPU Boost between both GPUs, raising the Power Target is the way to go. This will keep both GPUs at the same clock speed, and it won't be overclocking the GPUs. The GPUs occasionally spiked up to 1071MHz, after raising the Power Target, so it didn't affect maximum clock speed too much, it just helped "equalize" both GPUs to each other. So there is a easy tip for stabilizing performance with the GTX 690; raise the Power Target."