When you zoom in, the display centers the selection. If a time-range selection is smaller than the current display, the display adjusts to the range selection (or by a factor of two).
When you zoom out, the display centers the selection and adjusts by a factor of two.
When you're using a preset zoom factor (100% to 0.01%), the display centers the current selection and adjusts to the new factor.
There are various ways to zoom: