What makes SQL Server prefer page locking over row locking?
There are many reasons why page locks might be used over row locks, but the most basic answer is memory management. 400-500 row locks might be far less attractive than locking a smaller number of pages to accomplish the same task.

It's also possible that the way the query is written, or the way the index is designed is responsible for the type of locks taken.

See my posts here for greater detail:


Bad Estimates And Locking
Index Key Column Order And Locking