Terminal "history" command brings back different results

In OSX's terminal, I run the history command to find my recent commands. Often I notice recent commands will not show up, seemingly randomly. For example, I will log on one-day and notice my commands from the previous day are not showing in history (though they did they day before) but rather last week's commands are there. How do you troubleshoot this? Is it an error or I am understanding something incorrectly?