You can be right - my tests presently have been done on one book only. As soon as a python tool to get djvu from _jp2 will run with no human effort, I'll try it on lots of books to get some "general rule".
But - can you confirm that IA viewer shows jpg images coming from jp2-jpg folder?
Another problem, when using original IA pdf (again, I tested it on one book only: see https://it.wikisource.org/wiki/Indice:Tarchetti_-_Paolina.pdf
) is, that OCR text retrieved by mediawiki software is horrible in structure, please try to create any page of that Index. With pdftotext (xpdf) too, results are far from good.