You are not loosing anything
I believe two factors are at play:
1- The numbers themselves
Calculating a battery % used and kWh used is not trivial, there is no exact method to do that (that I know of).
So whatever method is used by GM to evaluate these number is likely imprecise. By how much I dont know, but it is there.
2- Imprecision in the reading itself
The number only show full percentage points, and the kWh only go to 1/10 of a kWh.
So when you look at a number that says 13%, it could be anything from 13% to 13.9%, and that for each of the 3 numbers, so if the there percentage number are being used, this carries a possible error of up to 2.7%!
In the same way when you read 13.3 kWh, it could be 13.1kWh, or 13.19 kWh
To eliminate one of the imprecision, you can write down the numbers when either the % or the kWh just changes to the next one up. At that time the imprecision is only on the other one.
For example:
- If you catch the kWh when it just changes to the new higher number, you know that one is right, but the % may be underestimated, which will lead to an extrapolated capacity that will be Over estimated.
ie: when the kWh just turns to 9 kWh, you note the percentages and they amount to 50% that would mean a capacity of 18kWh, but in fact the actual % could be as much as 52.7% which would mean a capacity of 17kWh
- If you catch the % when it just changes to a new higher number (assuming only the driving one is the only one), you will know the % is right but the kWh may be underestimated, which will lead to an underestimated battery capacity.
ie: when the % just turns to 50% (assuming only the driving one is there), you read a kWh of 9 kWh, that would mean a capacity of 18kWh, but in fact the actual kWh could be 9.09 which would mean a capacity of 18.2kWh
I have now gathered numbers every 5% for 5 charge and discharge cycles, and I ended up with numbers ranging from 18kWh to 15.8kWh.
That said, when I plot these numbers, I can see the trends. The Fluctuations get smaller when the number are read at higher percentage use (as the relative imprecision decreases), and I can deduct an average from these which is around16.4kWh
The way to do that is to make a reading when the battery is over 50% used, observing when the number changes to minimize reading errors.
Do that for at least 5 charge discharge cycles
Then put these numbers in Excel and create a line graph
Then use Xcel trend line feature to plot the trend and it will give you an idea of where the truth probably is.
Then forget about all that and start enjoying your car. Knowledge is important, but can be crippling if it becomes obsessive