When you remove the background from an image in Pages, there is a transparent bounding box around the remaining image. When you are using the Light Appearance mode, Pages shows the image styles with white behind that transparent background. If you switched to Dark appearance mode, you would only see the remaining image and a black background in the Style gallery.
With Light Appearance mode:

With Dark Appearance mode:

The one hold out is the last image in the Styles that represents what the image would look like if you chose the first picture frame example in the Border selection.
Tested: Pages v14.4 on macOS Sequoia v15.5.
Pages v14.4 for iPadOS v18.5 may behave differently as they are not the same application.