Here I have manually resized the image window by dragging the corner. After resizing the window, the scroll bars disappear (as expected).

As long as the rulers are turned on (in the preferences), this window zoom discrepancy (scroll bars appear after every change of image size) persists.

Turning off the rulers results in normal zooming of windows. No scroll bars appear after changes to soze of image (provided the image is small enough to display at 100% on my monitor).