Why is the voltage on my LCD not accurate sometimes?

TLDR; Much like a car gas tank, an estimated charge goes from full to half to empty. Your bike is powered by a battery bank made up of several small batteries in which the measurable change is small and it changes with temperature and power demand.

Your LCD shows an average voltage measurement, giving us a good reference value we can use. So knowing you charged your pack before a ride and your average mileage will help you get a better estimate. The value you see on your screen will be there every time you do the same procedure.

Details:

We added different visual aids to your LCD so the battery info is available to better help estimate your range. That being said, your setup will have a unique measurement that you will get easily accustomed to. Here’s why:

1. Your bike is powered by a battery bank made of several lithium-ion cells, similar in shape and size to a AA battery. This individual cell has an approximate voltage value of 3.7 volts. This value will have an average 1V variation between the 100% and 0% charged state which is 10 millivolts per each 10% of charge (these values will vary depending on cell manufacturer and battery model).
2. Available measurement devices are to be used as reference only, which means that they have not been recently calibrated to give you precise measurement. They will repeat the same measurement over and over so the offset value that might give is nothing to worry about.
3. If you use a voltmeter, remember it is not properly calibrated, and even though it repeats the measurements these will vary based on the device’s quality, usage, and even battery charge.
4. Once you power up your bike, there are several items consuming power, which will make a battery port measurement different from the one on your LCD screen.
5. While being used (charge-discharge) your pack will have a different internal temperature than when on standby or after finishing a ride. So a resting state will have a slight variation on measurement than on an active state. If you want to know your battery charge mid-ride, wait until you are on a complete stop to get a better estimate. This will not vary too much unless you were climbing a hill or going at full assist.
6. Your charger adjusts the charging output depending on your battery’s state. This is why we recommend to let the pack rest after a ride before charging it and also to charge indoors at a good temperature. 
7. Your charger stops charging at around 95% charge to prevent overcharging. This measurement is taken while loaded and active so it might not match an unplugged voltmeter check.
8. The measurements in your bike are taken by the battery BMS (battery management system) and your controller. These items are using the current-voltage level to function and as well can be slightly offset by the state of their communication connections.

We will continue improving the display and all functions which can lead to more precise measurements. But the overall behavior of the system will be the same

Link: Why does the voltage displayed varies while riding the bike?