This is persistence of vision. It's why those rotating led gizmos make it appear text is in thin air. Our brain is overlaying previous images on top of the current image. This provides a strange hybrid of the images in our brain. What you're perceiving has as much to do with speed of image change as it does with the relationship of the previous to the current image. The viewing distance between images also matters as this forces the image from our focus, into the peripheral, where our brain performs less processing and the nature of vision changes slightly (for example, use indirect peripheral sight at night to see better).