You need to understand Relevanssi does not understand language. It doesn’t know ”scalpel” and ”scalpels” are the same word. You search for ”scalpels”, so Relevanssi returns you the results you’re asking for, not something else. The search is working correctly.
Now, I understand you’d want ”scalpels” and ”scalpel” to be the same. There are a couple of solutions to this. Relevanssi Premium has support for a stemmer that can convert all words to their base stems, increasing recall (at the cost of reduced precision).
With the free Relevanssi, you could throw in a filter that modifies the search queries to remove the final ”s” from the search terms, so that ”scalpels” would search for ”scalpel”.
If you want a search that understands the nuances of natural language, look for products like Algolia.
Thanks for your reply and the suggestion you gave.