I can tell you how it works.
a) When the image height is smaller than the container height then the image is zoomed, proportionally, to reach the container height and centered horizontally. In your case that could happen for the woods image in some small viewports.
b) When the image height is higher than the container height then it means that the browser has zoomed it to reach the container width (in this case, the window width, and this still happens, 'cause you're using the full-width slider), then that option just center the slide vertically.
So probably you were viewing your site with a resolution that involved the case a).
Or maybe you've confused the vertical centering with a zoom. It may happen :)
Could you mark this topic as resolved?