Both are motivated by the perception that ABM technology may render ICBMs ineffective.
ICBM's are unneeded since the 80ies. That's why they don't have them.
The biggest advantage of a Skyfall missile over a Granit is obvious; range. If a Skyfall missile works as they plan, there'd be no need to sneak an Oscar anywhere close to the target, or even use a submarine at all. Furthermore it could strike inland targets beyond the range of a Granit fired from an Oscar sitting in coastal waters. An Oscar would need to sneak it's way up the Mississippi River if it wanted a chance of hitting a target in Nebraska with Granit missiles, which leads into:
ICBMs. With respect to ICBMs you're mistaken. Russia does have a variety of ICBMs and SLBMs still in service with plans for more. They have enough already to easily overwhelm America's extant ABM systems, which are primarily a defense against smaller nuclear threats (e.g. North Korea.) Poseidon and Skyfall are hedges against the possibility that ballistic missiles are rendered truly obsolete by ABM systems, but that scenario hasn't [yet] become reality.