I took a quick look at this before passing it along to the folks that understand CSS, and I think what's happening is the same glitch where tables in user content are inheriting the alternating-row CSS, it's just that your table only has one <td> in the spacer <tr> rows. The styling is applied to .tr .td, so only one <td> means only that <td> gets the styling.

If you edit that table so the blank <tr> has three blank <td> cells instead of just one, or to add "colspan=3" to the existing single <td>, does the grey extend?