Re: Plot twist? What plot twist?
Indeed.
If single phase power is 120V, three phase will come in at 208V.
Instead, homes usually have a centre tapped feed, giving a choice of 120V or 240V single phase. The latter is used for things like clothes dryers, water heaters, central air conditioning, and stoves.
Other things - stuff plugged into wall sockets, furnace motors, lights, sump pumps, garage doors, etc. are generally 120V.
As a result connection is simpler and more compact than 240V, with no need for things like fuses in appliance plugs and the option of designing for a two prong plug.
Also, it seems like 120V doesn't bite as hard as 240 - the few times I've had undue excitement with 120V, it wasn't too bad, but rather startling.
The 208V stuff shows up in some moderately heavier commercial and industrial installations like store refrigeration systems, large AC units, I susect for elevators, some computer gear, and so on.
The 'big stuff' may use higher voltages like 400, 600, or 800V... steel mills, motors moving 1100 tonne bridges, lock gates, subways, and the like generally go for higher voltages.
(And while it may be a bit odd, if you put me in the motor room of a big lift bridge, I will actually go over and read the specification label on the motors...)
As for frequencies, originally it was 110V at 25 Hz, but back somewhere around the 1930s, they decided that was inefficient and changed to 60 Hz - much better for transformers.
When I was a kid, antique radio gear sometimes showed up with 25Hz transformers, which were way bigger and could be used at much higher loads on the newer 60 Hz current.
Biting the hand that feeds IT
