The phrase "different ereaders" refers to various electronic devices designed for reading digital books. It suggests that there are various options available, each with its unique features, designs, and capabilities. Full definition
This means your eBook will be formatted to display consistently across a variety of different eReader devices, software and apps. (bookcovercafe.com)
I hope the market will produce different eReader form factors rather than the typical 6 ″ size. (goodereader.com)
Checking your ebook on the Amazon previewer located on your book's product page is especially important because it's common to have an ebook that looks fine on different ereaders, but that looks bad on Amazon. (mybookcave.com)