Matte layer not syncing?

Deke

Because I comp animation layers in After Effects, I often render a separate matt layer of just a figure. The setting and the lights are turned off so I just have a black matte. The odd things is that this layer is sometimes out of sync with the full render.  The matte is in sync with the character in the full shot, but goes slightly out of alignment every so often. What could cause this?  The full render is in Iray, but the matte is quickly rendered in Viewport.