为什么整个表被锁定而在更新语句中使用“with（rowlock）”

I update one row of a table,using WITH (ROWLOCK), but by executing "sp_lock" I can see that the entire table is locked. So, before the transaction is committed, other transactions can not update the other rows of the table. why "WITH (ROWLOCK)" doesn't take effect?I update one row of a table,using WITH (ROWLOCK