I am sorry for English, but my German is really bad.
There is a video on carwow where 58/62 kWh ID.3 drove 203 miles till 0%/0 miles and another 7 miles, to 210 miles, below 0%.
Capacity is declared as 58/62 = 93.5% usable. So there is a 6.5% buffer, let's assume 3.25% upper and 3.25% lower.
Car drove 7 miles below 0%, which is 3.45% of tracked usable range of 203 miles (7/203 = 0.0345). It is very close to 3.25% estimated lower buffer size, and the fact that it is a bit more could be explained by the fact, that energy cosumption during last 7 miles was clearly lower than the average due to reduced speed.
45/55 kWh ID.3 version has 19.2% buffer. Let's assume that 9.6% is upper and 9.6% is lower buffer.
Hypothesis:
If 45/55 kWh version can normally drive 250 km, then below 0% it could still be able to drive another 24 km (250*0.096=24), which is quite significant.
Is anyone with 45/55 kWh version up for a task to test range below 0%?
Ich habe definitiv eine 55-kWh-Batterie in meinem pure performance city, dieses video ist ein beweis dafür, das eine 48-kWh-Version existiert (oder existierte)
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(Bitte entschuldigen Sie mein schlechtes Deutsch)
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I would say they planned to use 48 kWh battery and car was designed like that, but they had to add an extra module possibly because they experienced charging errors on some chargers due to low voltage. That is why they recycle 48 kWh batteries from prototypes 
Just guessing.