Odd result in rendering animation matte

Deke

I'm using Daz for animation. Instead of rendering figure and background separately, I render them together in Iray. Then I delete the background, turn out all the lights, and render a silhouette matte layer in Viewport.

This typically works and I can use the matte in post to create a separate figure layer.  But frequently the matte rendered in Viewport is a bit off from the layer rendered in Iray.  The silhouette should match the figure exactly, but it doesn't. It's like the matte has been shot with a slightly different lens focal length. Do Viewport and Iray have different "lens" algorithms?