Thats cos of the revolution of earth
its actually is the same 12 hrs each but normally in summers the days r longer coz the earth is close to the sun at that time of the year and during winters the nights r longer and thats coz earth is not close to earth
The Earth's axis is not directly perpendicular to the sun..  The distance between the Sun and the northern hemisphere is close during summer and further during winter.. Thus the North faces longer days during the summer than what the summer faces. HOPE THIS ANSWER WAS HELPFUL