Which camera are you using? The Canon 7D records video and audio with a sync offset of 2-3 frames. PluralEyes will sync the audio to the exact frame, but the video will retain its original timing with respect to the audio.
If you are using Final Cut Pro and the sync offset varies across the duration of the clip, the cause is almost certainly an FCP setup issue as described here
PluralEyes is frame accurate under a very wide range of conditions. If what you are seeing is different from the above, we would be very interested in looking into it. Send us the project and log files as described here
and we'll investigate. Please copy or link to this forum post so we know what it's about.