As Venus orbits the Sun, it would follow an elliptical orbit, but only from the point of view of the Sun. Because Earth also orbits the Sun, the two motions are combined, giving a different shape.
The orbits of Venus and Earth are almost a perfect 13:8 resonance, meaning that Venus does 13 orbits for every 8 of Earth. 13:8 is a succession in the Fibonacci Sequence, and the previous one is 8:5. Because of the time it takes for Earth to complete this orbit (8 years), Venus has moved in a sufficient manner to trace a pentagram with curved sides.
From the point of view from Earth, every planet has an irregular orbit, because we are coupling the motion of Earth to the motion of the other planet. Venus, due to its distance, has the most pronounced irregularity, and due to the very close resonance orbit, it traces an almost perfect geometrical shape. It's pure coincidence.