iTunes devise summary shows more GB of audio synced than I have in my library

When syncing my devises to iTunes via wifi, the summary for each devise shows 15GB of audio synced to the devise. Problem is, I only have 12 GB of audio in my iTunes liabrary. (sync music video is not checked, so that cannot be the issue) If i connect each devise directly to my Mac via USB wire, the summary jumps back down to the correct 12GB of audio on each devise? This happens with both my iPad and iPhone, the discrepancy does not occour with my iPod Touch for some reason. All software is up to date on all devises. This has been going on ever since wifi sync was available. Seems to only be happening with audio and not Apps, Video, etc... Thank you