为什么在MySQL中使用Having子句时会跳过序列号

Question

我的查询：

    Select 
        @a:=@a+1 serial_number,
        CONCAT_WS(' ',ap.FirstName,ap.LastName) as Name,
        Count(t.ID) as NoOfBookings
    from 
        tablereservation t, 
        appusers ap , 
        (SELECT @a:= 0) AS a 
    where 
        t.AppUserID = ap.ID 
        AND
        t.RestaurantID = p_RestaurantID 
    GROUP BY CONCAT_WS(' ',ap.FirstName,ap.LastName) 
    HAVING Count(t.ID)=1
    Order by serial_number; 

结果：

我希望序列号从1开始，但是从5开始。

TableReservation表：

用户表：