Jack puzzled by appendix scar, has son named David

We start out the episode "Lighthouse" with one of Lost's signature bad Photoshop jobs. This one features the Shephard family, Christian, Jack and Margo. Next to it is a photo of Jack and Christian at the hospital looking very happy together, which is the opposite of what we've usually seen when they are together.

Jack rushes into his apartment to change out of his scrubs, and while he's in the bathroom, he notices a scar from having his appendix removed. Jack's mother Margo calls and asks if there's an update on Christian's coffin. Jack says that Oceanic told him that the coffin may have been checked through Berlin, Germany. He then asks her when he had his appendix taken out. She says that it was when he was 7 or 8, but in the original timeline, Jack had his appendix taken out on the island by Juliet.

Jack rushes off to St. Mary's Academy where a young boy his sitting on the steps waiting. We learn that this is David, Jack's son. So this indicates a massive change in Jack's timeline. This shows that his timeline has been altered not only 15 years ago with the birth of David, but when he was a child if he had is appendix out. So what are the causes of these big differences?