Here you can see that the lines on the Batim are not where the actual separation is for the compartment. It was done like this because the separation of the compartments was not straight and it doesn't look nice. So they simply painted over the separation and etched into the paint a straight line. This is a classic example of fake indentations. the Halacha requires that the Shel Rosh have four compartments and each one should be identifiable from the outside. Painting over the separation and making a fake line where they don't actually separate makes the Tefillin invalid. This is one of the many reasons it is important to purchase your Tefillin from a reputable source. To make the Tefillin Kosher they need to be opened and separated.

It's nice for the compartments to be straight and symmetrical but it is more important that they be Kosher.