Indeed, for some researchers the values are considerably different, for others the differences are not so large. See these discussions for considerations about the reasons:
I have heard of this problem many times previously. Most people say their h-index is consistently higher with GS. How much of a difference was there for you?
There are several reasons for the difference in the H-index and citation count between Google Scholar and ResearchGate. These some of the reasons:
1- Google Scholar may include more sources, such as books and conference proceedings, while ResearchGate may focus more on journal articles.
2- ResearchGate allows researchers to manually add their publications and citations, which can lead to inaccuracies if not all researchers report their publications and citations. Google Scholar, on the other hand, automatically tracks citations from various sources.