SQL can use more memory than what you set it to, as certain things like extended stored procedures, linked servers and other aspects of the service use memory outside that of what you set in the configuration options which generally control the buffer and proc cache's along with others portions of the process.

If lock memory is increasing then your serving up more locks, take a look at sys.dm_tran_locks to see if the number of transactions is increasing periodically that will give you a better understanding as to if its load increasing the lock memory, but <400MB out of 28GB doesn't seem like a concern.