A new Google Groups thread has some more insight into how Google determines an account's keyword limit.
Google AdWords representative, AdWordsPro, explained it can vary based on both "tenure of the account" and the success of the advertiser to utilize the keywords in his or her account over time.
Most new accounts get a limit of about "15,000 keywords for the entire account, inclusive of all campaigns," said AdWordsPro. But that can be increased based on the criteria mentioned above.
In short, keyword limits are set by "account level" and can vary greatly based on that information.
Forum discussion at Google Groups.