Why in the USSR sailors of ralves served longer than land?

Why in the USSR sailors of ralves served longer than land soldiers? The Navy of the Russian Federation is officially considered the successor to the Navy of the Soviet Union. Its main task is to ensure the security of our state. It is noteworthy that in the era of the USSR, sailors served in the Navy much longer than soldiers of the ground forces. What causes such "injustice"? In the USSR, any military swimming agent required high -quality specific service. The fleet service took longer, namely, 5 years (1939-1949) mainly due to the large amount of time necessary for the training of a specialist. For comparison, in accordance with the Law of 1939, 2 years served in the Ground Forces, and 3 in the border troops - 3.