They could have had an electric hot water tank that fed both units at one point, and then went to on demand hot through the boiler. The paperwork might just be old. Some older homes that have been turned into rentals have some crazy things going on. My panel feeds the on-property garage sub panel for example. I worked out a deal with my landlord over this, but technically it's a no-no.

It could also just be a regular old boring error, and only exists for units with electric hot water heaters that are included for some reason, like heat / electricity included units.

Basically, you got on-demand hot through oil, it is what it is.

I had a friend who did this when we were in high school. I will say the they end up losing a lot of their day if they really are going "home" at the end of the day.

Most of the time he stayed in town, but he would have gaps where he had to go back and ended up with a commute as a kid and the loss of time. Made him a great driver though...