Sorry, are you saying it drifted down to 11.8 or was that what you say immediately after a resynch? Assuming the latter, most likely, I'd say, it synched at 12 and then by the time you (or routerstats) got around to looking at it a fraction of a second later, it had dropped to 11.8.
If your tweaked margin has moved from 9 to 11.8, then it sounds to me like DLM has put you from a 12dB margin to 15dB.
I'm not familiar with how the GUI tools adjust the margin as I'm a dedicated luddite and only happy at the command line. With the DG834GT, the maximum variation via CLI is -6dB relative to DLM target, so I could speculate that 60% is something around -3dB. That's with the Netgear f/w, I'm sure somebody will clarify if the DGTeam is different, or if my speculation about GUI % adjustments is wrong.