That's a common problem with concluding things from automated literature analysis. Often the decision to scan only abstracts isn't an intentional experimental-design decision, but made due to "data of convenience": the researcher has easy access to a machine-processable set of plaintext abstracts, but not easy access to a similarly easy to work with set of full papers. Therefore, abstracts are analyzed!
Another potential confound for longer-term analyses is that the form of abstracts is not constant over the years: abstracts in the 1970s and 2010s aren't written in the same ways, and have different norms for what to include and how to include it. Among other things, the form of abstracts has gotten somewhat more structured/boilerplate, which is one reason I suspect they are finding an increase in all hits for their boilerplate search query.
Another potential confound for longer-term analyses is that the form of abstracts is not constant over the years: abstracts in the 1970s and 2010s aren't written in the same ways, and have different norms for what to include and how to include it. Among other things, the form of abstracts has gotten somewhat more structured/boilerplate, which is one reason I suspect they are finding an increase in all hits for their boilerplate search query.