I have an issue about a deadlock, which happens very rarely, and I would like to know what could be the cause here.

The message is this:

"Transaction (Process ID 56) was deadlocked on lock resources with another process and has been chosen as the deadlock victim. Rerun the transaction."

This is the command:

DECLARE @LastAccess DateTime
DECLARE @Numero Int

SET @LastAccess='2014-10-10 21:07:45'
SET @Numero=24349

UPDATE Member SET LastAccess=@LastAccess WHERE Numero=@Numero

This update on the Member table can only be done by one person. So, there cannot be two updates done on that record at the same time. Why do I end up with a deadlock on some isolated situations?