The Earth’s orbit is an ellipse, not a circle, and therefore the Earth speeds up or slows down depending on where on its orbit it is at the time. In turn this means that the duration of the solar day fluctuates from day to day, from a bit under 24h to a bit over 24h and back.

So if you take a picture every 24h precisely the sun will appear to move horizontally a little bit on top of the expected vertical movement.