Flow Illustrator is optimized for speed and robustness, with accuracy being traded in. Treat the videos as artist impressions. The artist is you.
In fact, Flow Illustrator does solve the Navier-Stokes equations governing fluid motion. However, the solution it obtains may be quite different from the real flow. This happens for two reasons. First, the picture, and the flow in it, is two-dimensional, while real objects are always three-dimensional. The difference can be substantial. Second, the Navier-Stokes equations are solved only approximately, and the approximation is good only provided that the pixel Reynolds number is small as compared to 1. The pixel Reynolds number Repix is the product of the velocity far upstream and the distance between points corresponding to neighboring pixels, divided by the kinematic viscosity of the fluid. (For example, if the picture is a bitmap 640×480 and the distance between the top and the bottom in the picture is 0.65 meters, the wind velocity far upstream is 7 meters per second, and the air kinematic viscosity is 0.000015 square meters per second (this is realistic) then Repix=7 * (0.65/480)/0.000015=631.94, that is much greater than 1.)
For the majority of real flows Repix will be noticeably greater than 1, and the calculations are very inaccurate.
Fortunately, for reasons far beyond what can be explained here, the calculation errors behave in such a way that, apart from small details, the flow in the video at a moderate Re might look quite similar to how the real flow would look at a much higher Re. Hence, an experienced fluid dynamicist, knowing what the flow should look like, can adjust Re in Flow Illustrator until the video looks just right.
Or simply play with the parameters and have fun.