Asking about water bill

We (I, my wife and my baby) moved to Higashi-Hiroshima on 15 Nov 2023, and registered for the use of water on this date. On 19 Jan 2024, I received my first water bill but I am a bit confused. On the bill letter, it was stated like this (I just translated it using Google Translate):
- Billing period = Nov 2023 - Jan 2024
- Usage period = 15 Nov 2023 - 7 Dec 2023
- Water usage = 8 m3 (water) and 8 m3 (sewerage)
- Basic charge (incl. tax) = 4,740 Yen (water) and 3,300 Yen (sewerage)
- Excess charge (incl. tax) = 0 Yen (water) and 0 Yen (sewerage)
- Total payment (incl. tax) = 8,040 Yen

However, in price list, it was written like this:
- Price list for 2 months
- Basic charge = up to 20 m3 (water) and up to 20 m3 (sewerage)
- Basic charge (incl. tax) = 3,160 Yen (water) and 2,200 Yen (sewerage) = 5,360 Yen
- Excess charge = over 20 m3 up to 40 m3
- Excess charge = 226 Yen / m3 (water) and 172 Yen / m3 (sewerage)

Now, what I do not understand is: Why should I pay 8,040 Yen?
I can still understand if the company charges me even for 2 months (5,360 Yen) as it may be because I came in the middle of a month.

Do you have any idea on this issue or have you ever experienced the same before? Thank you.

I don't live in Japan, but from what's written on your bill, what I understand is :
- one calendar month costs 2,680
- they check your consumption every two months to see if you used more than 20m3
- you were billed for 3 (calendar) months -> 8,040

My guess is that your next bill will arrive in March, where you'll pay for the full months of Feb+March (5,360), but your consumption will be measured between Dec 8th-Feb 7th.

you just pay the 3 months basic charges: 1580X3 + 1100X3 from November to January.

使用開始届の受付時期によっては２回分の請求を合算して請求することがあります。
the checked date was Dec. 7.
probably, you need to pay the full amount of basic charge for November.
when you received the first bill in January, it contains the basic charges for November, December, and January plus the excess charge until December 7th(this time the excess charge is zero.).

when you receive the second bill in March, it contains the basic charges for February and March plus the excess charges from December 7th to February around 7th.

The math makes it obvious the billed amounts are 1.5 times the two month charge. Many services charge a full month even when starting part way through a month. You are simply paying the charges for three months. The date of the meter reading is not relevant.
(I used to run a retail utility business - retail electricity - in Australia. Billing questions were very common.)