The scenario you are trying to achieve is somewhat strange.
Generally, when executing a mail merge, each record is placed in a new section or is at least separated from the previous (in custom scenarios). That's the default behavior and is the expected one.
What is more, the logic you've implemented inserts a fragment that contains a whole table, so it is only natural for each table to be separate. Generally, RadDocument's structure does not allow adjacent tables, so they cannot be added without the additional paragraphs. Nonetheless, implementing a mail merge as the one explained is not one of the scenarios supported out of the box and you can verify the same behavior if you try it in MS Word.
What I believe will much better suit your need is to build your table with the actual values instead. As you are building your document from code, you can simply add all records to a table, putting each in a single row, and add to the RadDocument this table instead.
I hope this helps!