Why does "fit to stage" only work sometimes?

[user370955456]

Adobe Captivate 2017: 10.0.0.192

MacBook Pro late 2013. macOS Sierra: 10.12.5

For some reason, the "Fit to Stage" button is greyed out some of the time. Sometimes images such as .jpg or .png will conveniently allow me to "Fit to Stage", and other times for images that are also .jpg or .png: the "Fit to Stage" button is greyed out. Why is that? Is there some type of image that this Captivate feature doesn't work with or something?

[Lilybiri]

AFAIK Fit to stage only works when the image is bigger than the stage, at least in one direction.