There are numerous factors that can cause this. For example any one or more of:
• the before/after space between paragraphs isn't an exact multiple the paragraph line spacing;
• widow/orphan control where a paragraph would otherwise only have one line on the next page;
• headings and the like that use a different font and/or point size than other paragraphs;
• the presence of footnotes.
In your attachment's case, it's the combination of your heading point sizes on page 4 and their before/after space. If, for example, you reduce the 'space after' setting for the 'Chapter heads' Style to 15pt, or reducing its line-spacing to 1, the gap at the bottom of that page will be reduced.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]