I think these are bugs. They are certainly unexpected behaviour!
Bug #1
Click on the zoom value (100%), type the number 9 (without the % sign) then use the mouse wheel to scroll up or down. (I am using a Logitech MX620 mouse)
Bug: Zooms to 3200%
Expected: Zooms to 8% or 10% (which is what happens when the zoom starts at 9% and ctrl+scroll up/down performed)
Other values exhibit the same behaviour (e.g. 3 & 44), whilst other values jump to different zoom values, e.g. 1 => 100% (sort of understandable, but not consistent), 2 => 2400%, 4 => 400%...
Bug #2
Click on the zoom value, enter a new value but don't press Enter or any other key. Move the mouse cursor over the image thumbnail in the top-right corner of the screen
Bug: Zoom value applied
Expected: Nothing