When I measure from hot A to ground of my 240V receptacle, I see 125.3V on my multimeter. And 125.8V from hot B to ground.
But across hots A and B, I see 216.7V. (There is no load when I'm measuring.)
I think I have single phase service, so shouldn't I be seeing something closer to 250V? At first I thought I was missing something regarding RMS voltage, but I'm fairly certain my multimeter is only measuring RMS voltage to begin with.
There are two main voltage setups for residential use in the US:
This is the most common. The 120V neutral is the midpoint on a transformer between the two 240V ends. Really simple and easy to understand. But the actual voltages have crept up over the years, so most equipment has been actually rated for 125V or 250V, much as there are still people referring to 110V and 220V. Higher voltages are ever so slightly more efficient, so there has been a gradual shift over the last century from 100V to 110V to 115V to 120V.
Even though many people will refer to 240V (or 220V or 250V) as two phases, that is incorrect. This is all a single-phase system, with the two ends (240V) being the two legs and one leg with neutral = 120V.
This is when three-phase power is distributed with neutral being 120V to any of the three hot wires. This is common in large buildings, so it is often found in large apartment buildings, though technically even a small building could have power fed this way. The ratio of 120V to 208V is exactly the same as 125V to 217V.
The good news is that you really don't need to worry about this very much. Most large appliances (water heaters, clothes dryers, ovens, EV charging equipment) is designed to handle a range from 200V to 250V. This equipment won't heat/dry/cook/charge as fast at 208V as at 240V, though in your case 217V actually gets it pretty darn close to the old standard of 220V.
You don't have a single-phase (or split-phase) feed, as you thought. Your service is actually two phases out of a three-phase feed, which is fairly rare but found in a few places including New York City apartments and various commercial buildings.
Years ago, service voltages were set at 120V/240V nominal (after many steps of upgrades from 100V at the dawn of electric service) for split phase and 120V/208V for three-phase. Utilities have been slowly increasing their delivered voltage towards 125V/250V (to be able to deliver more power on existing service wires) for split phase. The resulting phase-to-phase voltage in a three-phase system with 125V phase-to-neutral, is 217V (125V times √3).
This is normal and within spec. There is no fault.