Similarly to what the other have already said, I am surprised that A1 Level words constitute 61% of the words picked up by the Oxford Text Checker. But as Catherine said, would be interesting to use the same kind of tool but for phrases rather than words and check the results then.
I believe doing a similar activity with my students would be really useful to some of them. I think it would make them more aware of what words/phrases they use. It would also help them revise/learn correct collocations frequently used in academic English.