The "expanded" reference likely is attributed to contested origins of that final chapter. Some Biblical archaeologists claim that chapter 16 ended at verse 8. This means the Gospel of Mark would have ended with the verse that "Trembling and bewildered, the women went out and fled from the tomb. They said nothing to anyone, because they were afraid."
Mark is thought to be the first of the four Gospels to be written after the death and resurrection of Christ. Yet this account of Christ is missing many elements central to the Gospel. Joseph is never mentioned. The virgin birth is missing. And there is no appearance of Christ following the miracle of the empty tomb.
Thus, some centuries later, verses 9-20 were added to Mark's gospel. In many editions, these verses appear in italics to designate their addition to the original text. This addition is an "expansion" to Mark's Gospel.
Mark 4 contains several of Christ's parables, and He does tell His disciples that He speaks in parables so that those who truly seek the Truth will be given clear understanding. This first relies on faith in Jesus as the One who delivers the Truth.
As it turns out, His disciples initially lack the faith required for true understanding. This is fully revealed at the end of chapter 4 when they are in a boat and a storm blows up while Christ sleeps:
A furious squall came up, and the waves broke over the boat, so that it was nearly swamped. Jesus was in the stern, sleeping on a cushion. The disciples woke him and said to him, “Teacher, don’t you care if we drown?”
He got up, rebuked the wind and said to the waves, “Quiet! Be still!” Then the wind died down and it was completely calm.
He said to his disciples, “Why are you so afraid? Do you still have no faith?”
They were terrified and asked each other, “Who is this? Even the wind and the waves obey him!” (Mark 4:37-41)
The disciples are "terrified" of the display of power that Jesus has at His fingertips, the power to quieten the sea and winds. They shouldn't have been frightened, though. After all, they are his closest friends. They have been witness to the miracles He has performed. This lack of total faith in Christ has prevented them from fully understanding Christ's parables earlier in this chapter.