Because there are differences between papers included in paperpass and HowNet, papers not included in paperpass may be included in HowNet. This situation is very common. Many students get less than 20% data on paperpass, but more than 60% data on HowNet. These are common things.

Paperpass is sentence-based, and it can't recognize the contents, titles and references. Will be marked in red, and as you said, there will indeed be irrelevant articles that you have never copied. HowNet takes paragraphs as a unit, and can automatically identify contents, titles and references. The algorithm is also different. Therefore, the repetition rate of paperpass is definitely higher than HowNet.

In other words, paperpass is stricter than HowNet. The test with paperpass is 18%, and the school requires 20% with HowNet, which is sure to pass. Even if the reference is not pasted. Because HowNet doesn't take citation times as plagiarism rate.

Extended data:

Due to the inconsistent citation format, nonstandard reference format and false citation, PaperPass will no longer distinguish between "similarity" and "citation" in order to provide users with the strictest and most responsible test results. PaperPass detection system is a tool to provide users with the basis for revision in paper revision. Whether the similar parts (in red) displayed in the system belong to normal quotation will be left to the user for approval.

In addition, PaperPass is the largest paper inspection technology provider in China, and has provided technical solutions for many institutions and inspection systems. Therefore, after passing the detection of PaperPass, it can pass the detection of other detection systems.

Baidu encyclopedia -PaperPass