When adding a table to an entry, if the form is set to type xHTML, then the table will appear far below the entry content.  The reason for this is that the field turns all line breaks into <br /> and all double line breaks to <p>  This causes the table to render many lines below the entry.

There are two ways to get around this.  You can set the custom field to type: none.  The other option is to put all of your table code on one line.