I understand this one. The obvious answer is that you caught a glimpse, but it is more than that. I sometimes see it as a superpower when one of my mates is zooming through their text messages and I manage to read a whole text, despite it only being in my sight for a microsecond.
It definitely goes beyond our brains reading power, its probably more to do with a physiological aspect of our eyes. What I'm about to suggest is just a hypothesis, I haven't done any actual research on this.
A known fact is that our eyes do not track smoothly. If you take a video of your eyes and try to move them smoothly from left to right (without focusing on an object), it will fail. And you will notice that our eyes actually jump from focus point to focus point. This jump is called a saccade. Our eyes actually rotate at a peak angular velocity of 700°/s. Such a fast rotation causes our vision to be significantly blurred during this phase. My theory is: if you're scrolling really fast on your phone, or flipping pages of a book really fast, while your eyes are in a saccade movement - completely randomly your eyes reach a focal point with the blur on the screen or blur on the pages whizzing past. And our eyes take in this information and hold it for a bit longer because there is nothing else to focus on. Similar to when you hold your hand in front of your eyes in a well-lit room and then turn off the lights, and still see your hands.
Anyway, this is a confusing one. I am almost definitely wrong so don't take my word for it, but doesn't hurt to think.