They design everything to have an extremely high chance of surviving the mission life at full operational capability. If they design each system to have a 50% chance of surviving 2 years, and there are 5 systems, then there’s only a 3% chance the mission will actually make it to 2 years.

Even then, if NASA wants 2 full years, then they’ll shoot for a high likelihood of meeting it, like 95+%. That means there’s a decent chance it’ll make it well past 2 years.