Overkill is a misunderstood expression. It just means “redundancy because of failure.” That is, nuclear missiles are *very* prone to failure.
They blow up in their silo during launch. They fall apart just after leaving their silo. They break up during ascent while under enormous stress. Their guidance fails and they go far off course. Then their warhead fails for one of many reasons. And lots of things disrupt them.
American missiles are the best in the world, but we still have very high overkill. Russia’s overkill is so high that it far exceeds 50% of their launches.
Tactics also matter. Russia makes it a point to target enemy military targets. The US targets cities. Russia also doesn’t maintain their equipment. For example, near the end of the Soviet Union, the US Navy derisively called the Soviet Navy a “naval hazard”, because their ships were in such poor repair.
And nuclear missiles need one hell of a lot of maintenance.
Actually, due to poor guidance and reliability , The Soviets/Russia target population centers, the US has great reliability and accuracy, therefore we target military capability and missile sites.
Our missiles are “old” but due to constant component upgrades and such and while the missiles are “old,” they are “new.”