Why did Harry see flashbacks when Snape used legilimency on him?
As we all know, Harry was taking occlumency in Harry Potter and the Order of the Phoenix with Snape. When Snape casted legilimens, Harry's mind was like having a flashback or something he himself saw... Snape's the only one supposed to divulge into his past.
“I am about to attempt to break into your mind,” said Snape softly. “We are going to see how well you resist. I have been told that you have already shown aptitude at resisting the Imperius Curse. . . . You will find that similar powers are needed for this. . . . Brace yourself, now. . . . Legilimens!” Snape had struck before Harry was ready, before Harry had even begun to summon any force of resistance: the office swam in front of his eyes and vanished, image after image was racing through his mind like a flickering film so vivid it blinded him to his surroundings . . He was five, watching Dudley riding a new red bicycle, and his heart was bursting with jealousy. . . . He was nine, and Ripper the bulldog was chasing him up a tree and the Dursleys were laughing below on the lawn. . . . He was sitting under the Sorting Hat, and it was telling him he would do well in Slytherin. . . . Hermione was lying in the hospital wing, her face covered with thick black hair. . . . A hundred dementors were closing in on him beside the dark lake. . . . Cho Chang was drawing nearer to him under the mistletoe . . . No, said a voice in Harry’s head, as the memory of Cho drew nearer, you’re not watching that, you’re not watching it, it’s private — He felt a sharp pain in his knee. Snape’s office had come back into view and he realized that he had fallen to the floor; one of his knees had collided painfully with the leg of Snape’s desk.
(Harry Potter and the Order of the Phoenix CHAPTER 24: OCCLUMENCY)
When Voldemort read Harry's mind in Harry Potter and the Philosopher's Stone, Harry didn't feel even a single twitch or flashback in his mind. (Voldemort read Harry's mind to know where the stone is)
Is there a difference between these two?