This not only included proofs and essays, but perforating wheels and other metal as well, all of which they sold off as scrap (almost always to stamp dealers). Back in the 1890's double perf varieties were very popular, and there were certainly people ready and willing to help create a supply to fill the demand.
Just like in Baseball,they are errors. No need to grade them, they have been graded as an error by the perforator.
Well, not quite Stuart, Some of them are actually double perforated in the normal course of production. But their popularity increased their availability when the perforation equipment became available. Telling the true production errors from the later "post production" modified stamps would be probematic at best. Some are true errors, while others are manufactured errors.