The underlying pagination modules for HTML and PDF/Word are different, and so the number of pages in a report will vary depending on the format used.

In HTML format. soft break algorithms are used.  You can influence the vertical page breaks using the interactive height property in RDL, but these are just hints to the pagination algorithm.

In PDF/Word format, hard (physical) page break algorithms are used, as the size of the single page is fixed.  In addition, there are RDL properties such as KeepTogether to allow you to control how report items are placed on a physical page.