If the SOC is based solely on current in/out of the battery, the first couple of percent will last longer than the last couple of percent due to the difference in voltage (higher voltage = less current at the same power). The linearity of the battery gauge would follow this graph with higher voltage meaning more range per %.
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It's also totally possible though that MINI is compensating for the voltage difference making the gauge pretty much linear. Either way, the difference is mostly negligible for 95% of the pack
