The key is that which X chromosome is deactivated is not the same in all cells.
It does occurs fairly early in the development process (a stage called gastrulation), and the deactivation does persist for all cells that descend from a given cell present. But critically, it's late enough that a retina could conceivably possess cells descending from two different "lineages".
You can actually visibly see the "resolution" of the deactivation by looking at tortoiseshell cats. Each blotch of orange/black represents one cell present at the deactivation stage.
It does occurs fairly early in the development process (a stage called gastrulation), and the deactivation does persist for all cells that descend from a given cell present. But critically, it's late enough that a retina could conceivably possess cells descending from two different "lineages".
You can actually visibly see the "resolution" of the deactivation by looking at tortoiseshell cats. Each blotch of orange/black represents one cell present at the deactivation stage.