Odd behavior with zooming

I was testing out some camera zooms as alternatives to fadein/outs, and noticed some odd behavior when zooming in, that really is only evident with large changes in zoom (it started getting noticeable at about 25x zoom):

Basically, the zoom works. It accelerates, then decelerates. No problems there. When you first call startCameraMove, however, the zoom "jumps" in a bit (more and more noticeable with larger zoomtargets). Zooming back out likewise causes a jump, but this time at the end of the zoom move.

I'm not sure if I'm setting things up properly, or if it's a bug in the camerazoom code (I'll try and take a look if I have time today), or if it's simply not setup to handle large moves like this.

Here's the relevant code, for reference:

playerMap.bindCmd(keyboard, "[", "playerzoomOut();", "");
playerMap.bindCmd(keyboard, "]", "playerzoomIn();", "");

function playerzoomIn()
{
	%z = 1000 * sceneWindow2D.getCurrentCameraZoom();
	sceneWindow2D.setTargetCameraZoom(%z);
	sceneWindow2D.startCameraMove(5);
}

function playerzoomOut()
{
	%z = 0.001 * sceneWindow2D.getCurrentCameraZoom();
	sceneWindow2D.setTargetCameraZoom(%z);
	sceneWindow2D.startCameraMove(5);
}

edit: oh, and in case it wasn't clear, the interpolation mode is set to Sigmoid. The initial jump doesn't happen with linear interpolation. So I'll be looking at the appropriate part of the code in the meantime.