There is any number of possible explanations for this, including:
• One column having more headings than the other, or headings that span more lines than in the other, where the headings use different font sizes and/or before/after spacing than other paragraphs;
• different numbers of paragraphs, where the before spacing and/or after spacing for each paragraph doesn't equal the line height;
• 'widow/orphan' control;
• 'keep together' and/or 'keep with next' paragraph formatting.

Without actually seeing the problem document, it can be difficult for anyone to diagnose the issue. You can attach a document to a post via the paperclip symbol on the 'Go Advanced' tab at the bottom of this screen.

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