Sounds Good, it if reads higher than the rating on the water heater. It could always be losses in the line. I just fixed some really bad wiring in my house. Without going through the entire saga, there was over 500 of 14awg wire sent through out my house before it winded back out to the garage, where the meter box is… crazy I know… no run should be that long, but that is how they did it. It has been a problem forever, I could not even run a portable heater out there because the voltage was dropping as low as 70 volts. I out of curiosity with the sense meter turned the heater on to see what a watts up meter was saying vs what sense was showing the draw at the mains… holy cow, the difference was great. bottom line is I was losing 400+ watts in the lines. I don’t recall the exact number might have been a lot more, but the crazy part is I was pulling and paying for a whole lot more power from the mains then was actually being utilized at the portable heater. Which explains why have had electrical wall fires in the past. I solved the problem by running a new 15 amp circuit and used 12awg, instead of the borderline 14awg crap most contractors run for 15 amp circuits. mind you 14awg is fine if you keep the runs below 100 feet, but at 500+ feet, the power loss is insane.
Do you recall the length and gauge of your generator wire running to your mains?