This is often caused by invalid HTML syntax. Specifically, it is often caused by loose content within the table (i.e., content that is not inside a TD or TH element). There is no standard way to handle loose content within a table. Some browsers display all loose content before or after the table. When the loose content contains only multiple line breaks or empty paragraphs, then these browsers will display all this empty space before or after the table itself.
The solution is to fix the HTML syntax errors. All content within a table must be within a TD or TH element.