RPD came before RPK in real life
Ok, in 1945 the Soviet military fielded the RPD for the first time, chambering the 7.62x39 mm round, which was later fitted into the SKS, the AK-47, RPK, AKM, and Makarov PM. In the late '60s and '70s they replaced it in favor of the RPK, as the RPD had a tendency to jam due to the belt system built into its ammo box. The RPD still found use in the hands of communist guerillas who needed second-hand Soviet weaponry to fight their war (such as the Viet Cong and the Khmer Rogue), and in the hands of up-start Arab nations. By the '80s, however, most Arabic nations, such as Iraq, replaced older weaponry with new designs, such as the RPK-74. Now, Al-Qaeda and the Taliban use a combination of self-made AK-47's and RPK's in their arsenal.
That's it for the history lesson, but why did Infinity Ward/Treyarch decide to reverse the roles in their video games. In Call of Duty 4: Modern Warfare, the RPD is seen in the hands of Al Asad's OpFor and in the hands of Ultranationalist rebels, which, in terms of real life, both sides would be using RPKs instead, and possibly some RPD's in the hands of the OpFor. In Modern Warfare 2, the RPD is seen again, correctly used in the hands of the poor Brazillian Militia, but unrealistically as the main light machine gun for the High-Tech Russian Invasion force.
Reversed, the RPK appears unrealistically in the hands of Viet Cong Guerillas in Call of Duty: Black Ops. The Soviet Spetsnaz MIGHT be using RPKs, but the RPD would be most common.
And the AK-74u in Black Ops is probably as unrealistic as that game gets (they have the -74 suffix on the end to obviously show that it was made first in 1974. and Black Ops ends in 1969; a 5 year difference.)
Why do you think they did this?