Static objects (i.e. those that do not use any variables) are placed into overlays in order to simplify the spooled description of the design and increase processing speed. Overlays are created as the objects are processed in their Z-order. The static objects are merged into an overlay, one by one in that Z-order. However, if the next object is variable then it cannot be placed in the overlay and so that overlay is finished. The next static object in the Z-order will be placed in a new overlay and the process continues.
It is good practice to place static objects consecutively in the Z-order so that they will be processed as a single overlay. This can be done quickly via the Edit menu of Layout, select Optimize | Static Objects Order.