This is a known issue that has been with decklink cards for at least the 8 years that we have been using them.
The processing on the cards seems to delay the video and audio by 4 frames. This is why Fcp has a 4 frame delay built into their setup, to delay the computer monitor video by the same 4 frames, so that it matches your broadcast monitor.
In Fcp the 4 frame delay setting is under system settings and then select the playback tab…viola.
This keeps your playhead and your computer video in line with the decklink video/audio output.
In cs6 there doesn’t seem to be a delay solution that I can find.
Solution…buy a aja kona card.They are a little more expensive but have no noticeable delay.