Hi LauraGSpeaks. This is a complicated and complex question. The answer I offer may be taken in several hues of light. The rules HP complies with is both their own and those of additional organizations. Theirs are more "than" likely patterned from the more formal organizations.
To give you a hint I looked into a poem I wrote once that seems to have been copied in a paraphrased format. It became a book and did well. The logic of order and the meaning of the poems stanzas are very, very similar. The quick of it is by the time I finished researching it became a 48 page paper. Copyright law is very, very complex.
I submitted the paper to reliable resources who said they agreed with the concept & that the time line of both do correlate, however the cost to prove it was outweighed by the risk of achieving a positive result or a win in court. So, with the information gained I have it sitting on a back burner while that book does earn from what I feel is my poem.
The crux is there are two or more means available. The first and easiest is to prove there is an infraction of being plagiarism. In essence plagiarism is simply fraud. A great resource for understanding this is plagiarism.org. Many learning institutions will dismiss a student from a class if caught willfully plagiarizing a work with a failing grade. The key word is "willfully." Some extend that penalty to expulsion. So, plagiarism is not treated lightly within academic & professional circles. Again, the keyword is "willfully."
I know I have written a hub, posted it, and then within moments seen a hub very close to what I produced. My dander was ruffled for sure, especially since the comments and etc indicated greater success with the other hub. However, my score was high, my readership was acceptable, and accolades offered well received. Yet, I unpublished mine because the other hub was published first. I felt like others would "Feel" with "perceptions" I plagiarized that hub. It is a matter of ethical choices.
The web, considering the web is on the internet, and there is more to the internet than the web, offers a plethora of both opportunities and the probability of the possibility of likeness. Kinda' like reporting on the presidents' speech is by hundreds of reporters. Many report from the news feeds too. Randomness alone offers similar will certainly ensue.
The key is marketing your work & gaining a higher readership offering validity as first & foremost. If damages occur then proceed.