Your content editor may not always reflect a particular format as it is being used to create all three different file formats.
Pressbooks is capabe of creating multiple formats simultaneously from the content that's in your chapter edtor.
In order to do this, the design of your book has a different style sheet (CSS) for each export format:
This allows users to modify and customize settings for each export type. This is important to support different affordances and accessibility considerations for each format.
For example, a user may choose to set their webbook paragraph separation setting to skipped lines but their PDF paragraph separation setting to indented paragraphs. Skipped lines make it easier to read text on a screen, whereas indented paragraphs are better for the legibility of printed documents.
These customizations can be made in Theme Options or in Custom Styles.