I’m not surprised by this. A nursing home could be considered to be a (very) large household.
Speaking for my own household, my son got it first. Then the rest of us got it.
The wife of a friend of mine had been C19 testing employees in a nearby meat packing plant. Testing them in their homes. Everyone in each house testing positive.
I’m only guessing, but I’d say that a nursing home that has cases, has many cases. Whereas one that doesn’t have cases, obviously doesn’t.
You could easily have two nursing homes on the same road with 100 residents each. One with zero cases, the other with 80.
IMHO.