I never did find the reason why this occurs, but I did find another piece of code that addresses the issue. Here is the relevant portion, in case it helps anyone else:
if (xStep <= 0.0f && VideoLiveWallpaper.xStep == 0) {
xStep = xOffset / 2; // originator didn't set step size, assume we're halfway
It seems to work as long as the home screen (middle screen) is up when the wallpaper starts. That's not perfect, but it works better than my method of assuming 5 screens (xStep = 0.25) in all cases when it's zero...