In the modern age, an act of plagiarism like this might be a lot easier to inadvertently commit than a lot of us give it credit for. I suspect what happens on some occasions is that a writer uses Notepad or some sort of word processing app to take notes. On some occasions, they paraphrase whatever nugget of information they come across. Other times, they find it easier to just copy and paste a graf from their browser with the thought that they will end up incorporating the information into their overall work. Unless the writer is super organized, I would think it would be easy for them to occasionally mistake a rather mundane block of text in their Notes document for their own paraphrasing, rather than a direct copy+paste.
Really, it doesn't make sense for a writer to knowingly crib such a mundane paragraph when they could easily rewrite that information. I guess that doesn't excuse it. And if the writer does not offer attribution, it is a clear sin. But if she cites the source of the information of a paragraph that ends up being lifted verbatim, we're really only talking about the lack of italics of quotation marks or an attributive parenthetical. Sloppy, yes. But a different ethical ballpark than knowingly passing off somebody else's work as one's own.