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Best I understand, you're running a 30:1 AFR under boost, so it isn't like you're reducing vacuum. I would guess that the extra gasses in the cylinder are acting as a medium of expansion, and are causing some thermal efficiency gains as well. Water injection can increase economy because of the expansion from the phase change, which more than offsets the energy lost to the phase change itself, just as an example. And, if you have twice as much air in a cylinder, and you add a given amount of energy to it (combusting a fixed amount of fuel) the temperature will be lower and you'll be losing less energy through the cylinder walls and combustion chamber. The turbocharger also harvests some of the expansion energy and puts it back to useful work.
Just an educated guess, however.
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