The length of ID numbers grow as needed. They are in chronological order with the exception of the first few thousand posts, users, and threads.
The early IDs are not in order because of our first forum software conversion from Ikonboard to vBulletin in late 2001. The conversion software, which imported the Ikonboard data into vBulletin's database, preserved the original dates but did not process the data chronologically. This is why the
first thread has an ID of 56, the first post in the first thread has an ID of 1222, and why my account has an ID of 658.
@Jordan's Other Account has the ID of '1' because it was the account I had to create during the vBulletin installation process (you had to install the board before you could import data from another platform into it). That account was not deleted during the import, so once that process was completed, I was left with two accounts. Account merging wasn't a feature in vBulletin back then, so I just kept using my original account... and I still am, 17 years later.