Two very similar sentences. 80-90% similarity word-wise. Meaning-wise? Completely different. That's why the human element is required. We can tell those two statements tell an entirely different story when read. These small similar sets of wording add up quite quickly, and a 30% similarity in your case, given the level of research probably done in whatever your field is, and the amount of sources you have probably cited (100+?) is unlikely to be anything to fret about in this day and age.
