Here's what I'm not getting at the moment. What are the connections to that transformer? If it is 3 phase leg to neutral on the primary side, then the center tap should be above neutral on the secondary (house) side, right? Which would mean that house neutral is NOT at ground level. But I'm pretty sure it is.

your confusion is becasue you don't understand how a transformer works. The pri and secondary are isolated from each other. The center tap of the sec has NO RELATIONSHIP to the primary. Think of the secondary as seperate floating power supply not related to the primary. The center tap of the secondary is neutral or ground ONLY becasue it gets connected to ground. Since it is connected to ground the two ends become +120 and -120 relative to neutral.

Mark