There are many places where Picture Window resets the zoom factor with a zoom to fit in order to make the entire image visible. This avoids mistakenly think you are seeing all of the image when in fact only part of it is being displayed.
While the current zoom to fit works well for photographic images, it make tiny images too small to see. While Picture Window was not designed to edit tiny images, for the next release I added a Preferences setting to make zoom to fit always resize the image to fill the image area even if it means magnifying it beyond 1:1. This lets you choose how you want it to work. Consequently, I removed the Shift-click on the zoom to fit button I added in the previous release.