Letterbox not 16:9 ratio?

[Redstreak]

I've been noticing for a while that the shape I see on my viewfinder in letterbox mode isn't actually 16:9, but I assumed that was just the viewfinder (I still think it is partly). I just logged some letterbox footage into Avid and stuck a 16:9 mask on it, and the mask covered up a little more of my video on the to and bottom - for some reason, the camera is letterboxing a different ratio than 16:9. Why is that? What is the ratio?

Thanks,
- David