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I can't speak for your specific vehicle, but engines under boost have higher cylinder pressures. To prevent detonation (also called "knock"), the typical strategies involve going rich and retarding ignition timing.
Going rich reduces combustion temperatures by dumping in extra fuel which won't be burnt to make power, but instead just absorbs heat and is flushed out the exhaust.
Retarding ignition timing starts combustion later, and effectively dumps part of the useful energy that could have been extracted from combustion, out the exhaust.
If your engine has the proper fuel and spark maps to take advantage of E100's higher knock resistance, it may not need to go rich or pull any timing under boost, but that isn't a given.
Turbo engines usually have lower static compression ratios, which makes it so they don't need to dump as much fuel or pull as much timing under boost, but it also makes them less efficient outside of boost.
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12-13km/L all-city driving doesn't seem bad at all to me, frankly, but there aren't many (any?) cars sold in the US which can match those figures without hypermiling, that aren't hybrids.
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