Why is the flight duration of the same flight different in different months?
Jet stream is a stream of air flowing around the Earth from West to East at high altitude. With wind speed averaging more than 100+ km/h, it has a tremendous effect on a plane's ground speed.

The pattern, strength and location of the stream is constantly changing. For example, during the (Northern Hemisphere) winter the stream generally strengthens, and during the summer it generally weakens. With modern technology such as weather satellites and data fusion from thousands of weather stations around the world, the stream is fairly predictable.

The effect is more pronounced on long flights because these flights stay in high altitude much longer.