The reason for those is hinted at the [first quote you'll see when starting the game][1].

Booker was transported through a tear to another universe, and his mind had to make up memories to deal with traveling to another universe.

Notice that Booker did not know what or where Columbia is (he had to ask the first man he saw in Columbia where he is), and has shown surprise on seeing a flying city. This is despite Columbia being a significant part of the United States (before its secession) and took part in a major battle of that period, the [Boxer Rebellion][2]. It is probably safe to say that Columbia did not exist in the universe Booker was from.

To make sense of this, Booker had to modify or create new memories to deal with being transported to another universe. [Another answer][3] has already mentioned that Robert Lutece (who was also transported to another universe before Booker has), has experienced this.

Another example of Booker making up new memories is when he was transported to the universe where he is a martyr of the Vox Populi. He then "remembered" that he is/was a hero in that universe and fought with Slate (instead of fighting against Slate in the previous universe).

In the end of the game, Booker finally completely "remembers" what really happened, when he was transferred back to the day of his baptism. This event was a turning point for Comstock/Booker. One took the baptism, and the other didn't. More details regarding the ending and the timelines as posted in [my answer on another question about the game's ending here][4].

[1]: https://i.sstatic.net/5qeAV.jpg ""The mind of the subject will desperately struggle to create memories where none exist..." -Barriers to Trans-Dimensional Travel, R.Lutece, 1889" [2]: https://en.wikipedia.org/wiki/Boxer_rebellion [3]: http://gaming.stackexchange.com/a/112141/4797 [4]: http://gaming.stackexchange.com/a/112976/4797

The reason for those is hinted at the [first quote you'll see when starting the game][1].

Booker was transported through a tear to another universe, and his mind had to make up memories to deal with traveling to another universe.

Notice that Booker did not know what or where Columbia is (he had to ask the first man he saw in Columbia where he is), and has shown surprise on seeing a flying city. This is despite Columbia being a significant part of the United States (before its secession) and took part in a major battle of that period, the [Boxer Rebellion][2]. It is probably safe to say that Columbia did not exist in the universe Booker was from.

To make sense of this, Booker had to modify or create new memories to deal with being transported to another universe. [Another answer][3] has already mentioned that Robert Lutece (who was also transported to another universe before Booker has), has experienced this.

Another example of Booker making up new memories is when he was transported to the universe where he is a martyr of the Vox Populi. He then "remembered" that he is/was a hero in that universe and fought with Slate (instead of fighting against Slate in the previous universe).

In the end of the game, Booker finally completely "remembers" what really happened, when he was transferred back to the day of his baptism. This event was a turning point for Comstock/Booker. One took the baptism, and the other didn't. More details regarding the ending and the timelines as posted in [my answer on another question about the game's ending here][4].

[1]: https://i.sstatic.net/5qeAV.jpg ""The mind of the subject will desperately struggle to create memories where none exist..." -Barriers to Trans-Dimensional Travel, R.Lutece, 1889" [2]: https://en.wikipedia.org/wiki/Boxer_rebellion [3]: https://gaming.stackexchange.com/a/112141/4797 [4]: https://gaming.stackexchange.com/a/112976/4797