Mysql Over() 函数不覆盖表中的所有行

我正在用MySQL练习SQL，在SQL中遇到了一个奇怪的行为.假设我有一张这样的桌子:

Delivery table:
+-------------+-------------+------------+-----------------------------+
| delivery_id | customer_id | order_date | customer_pref_delivery_date |
+-------------+-------------+------------+-----------------------------+
| 1           | 1           | 2019-08-01 | 2019-08-02                  |
| 2           | 5           | 2019-08-02 | 2019-08-02                  |
| 3           | 1           | 2019-08-11 | 2019-08-11                  |
| 4           | 3           | 2019-08-24 | 2019-08-26                  |
| 5           | 4           | 2019-08-21 | 2019-08-22                  |
| 6           | 2           | 2019-08-11 | 2019-08-13                  |
+-------------+-------------+------------+-----------------------------+

我的问题是:

SELECT COUNT(*) as imm, count(*) over() as all_t 
FROM 
Delivery
WHERE order_date = customer_pref_delivery_date 

结果:

+-----+-------+
| imm | all_t |
+-----+-------+
| 2   | 1     |
+-----+-------+

我希望over()函数将覆盖整个表，在本例中返回6，但出于某种原因，它只返回1.这种行为有什么解释吗？