You are right Megan. These two words do essentially have the same meaning. The reason why the author uses them both here, together with "guess", is very simple. This is part of an academic text, written by an academic, possibly in a university context, possibly a student - possibly for an academic paper presented in an academic journal. The authors of such texts/ papers/ articles are well known for using long complicated sentences with long obscure words to make their statements sound more important, and to impress others with their knowledge and learning. They want to make the reader work hard to understand the full meaning of their words. Because if you have to work hard to understand something, you probably imagine that it is very important, and that the only reason it is difficult for you, is because you are not as intelligent as the person who wrote it. I have read many academic text written in this style. The vast majority of them could be expressed with much simpler words and less space. It is a sad fact of life that they are not.

They don't have the same meanings. They have similar meanings, which is different. Using more than one word intensifies the statement and makes it more precise. It would sound very weak and unconvincing to use only the word "guessed" for example. The author is making a very strong claim.