It won't have a background of blue because you're entering invalid TDs. A TD cannot go inside another TD without it having a table supporting it first.
<td><td></td><td> doesn't work
<table><tr><td><table><tr><td></td></tr></table></td></tr></table> although ugly, works.
But we're not asking for extra TDs so I have no ideas where they're coming from as they weren't there before