Balance means that the recorded consumption amount does not include the unrecorded consumption or installment (generally speaking, it is arrears), a negative number means excess deposits, and a positive number means the amount that needs to be returned.

For example, yesterday was-12, which means that 12 yuan was added to your account when you inquired. Checking 987.48 today means that you need to repay 987.48 yuan. Maybe you will say that I didn't spend money in the meantime, why did the amount change? Because maybe you spent more than 1000 to find-12, but the money was not recorded in the account at that time, nor was it displayed in the balance field.