There is a difference in the mechanism of research adoption and the number of citations also, there is some research not calculated by Researchgate and vice versa...
I believe Google Scholar and Research Gate doesn’t share their latest Database with each other. Although older data showed the similarities of Citations of old Articles. Its a hard work for the Researcher to gather all the correct platforms Citations information. There are other platforms Pubmed index journals others too to collect all the Citations. Regards
I have more than 575 google scholar citations but only 242 on RG. This is also true of the h-index. Primarily, the search engine for RG is not as powerful in identiflying citations. In fact, even google scholar can’t find many of my citations because they are not in readable pdf format!
As well known, h-index calculation is based on citation of the publications,
her is two posable reasons for variation,
first, journals indexing
for instance, for Scopus if a journal is not in the list of indexing databases, it would not calculated,
second reason, a different author's name format,
if the author has used different styles for his name and this format is unknown for the indexing databases, it will absolutely affected the authors citations
Another difference is that RG also provides H-index excluding self citations, which Google Scholar does not provide. Nevertheless, in my opinion, citation count from Web of Knowledge is more precise indicator of a researcher's scholarly capabilities compared to both RG and Google Scholar.