I'm not sure if the tachometer is reading via the crankshaft position sensor, or what it's driven off of, but I do believe it would be measuring one rotation of the crankshaft (or flywheel) and noting that as a revolution.. not cutting it in half for each firing.
If you think about it, the engine is still doing a full revolution, there are just explosions in 3 cylinders at a time, but the parts are still moving, doing their job expelling the exhaust gases..
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