Add an ORDER BY clause. Data inside SQL Server isn't stored in any specific order, it's all a "heap". When you use a TOP 1, without an ORDER BY, the result you get back will simply be the first row in the dataset created; which could be anything. Adding an ORDER BY defines the order (surprise that!) the data is returned in, thus resulting in the TOP 1 being the first ordered row.

Try yourself, and post back if you get stuck.