There are a limited number of graphics in the app and there are some weather conditions where the app doesn't have a moon-type graphic to go with that particular condition, so the sun animation is shown, even if it is night. The “Passing Clouds” and "Haze" graphics are two examples of conditions that don't have a “night” version. We're looking at adding additional animations in a future version of the app.
The app also currently has fixed hours for sun/moon animations. It doesn't do any sort of calculation. Sun is shown from 7 a.m. to 7 p.m. during daylight saving and 6 a.m. to 6 p.m. during U.S. standard time. So there may be locations where the animation does not match, but the weather conditions should be correct.