More like if 1% of the rockets still work and 1% of the warheads still work, which equals 0.01%.

Keep in mind that you need to actively maintain warheads and the delivery vehicles.

Fissile materials have a half-life, and it only takes a few years for them to decay enough to make the warhead nonfunctional.

Rockets also decays in a similar manner. Fuel breaks down into various chemicals. Structural metals corrode, suffer metal fatigue and creep, and can also be broken down by chemical reactions with the atmosphere and the fuel.

And Russia just doesn't have the budget or expertise allocated to do a great job of maintaining their stockpiles