Right, it could literally be as dumb as: all the tests were processed by one person, and that person is infected with covid-19.

Drawing signal out of the noise right now is very, very difficult.

That's it, you nailed it.

What's more likely, a population that gets all infected all at the same time, or one person infected being careless with samples?

Then there's more: given how many tests of similar populations are getting done, what are the chances that some have a careless infected tester?

A cruise ship under quarantine is dramatically more socially isolating than a prison.

You would expect spread to be absurdly more efficient in a prison. Less physical separation, less hygiene, less everything.

Densely crowded environment where people were unlikely to maintain social distancing, poor quality health care, probably poor quality sanitation. It seems reasonable to me that a prison would fare worse than a cruise ship.

Not quite this huge, but then again states generally try to get them flu vaccines. It's reasonably common for prisons to enact emergency measures to stop the spread of the flu.