While research sites like google scholar, gives a number of citations for a given researchers' work, RG still provides him/ her a lower citation record.....Also, this sounds the same for the calculation of h-index.
It depends on the different algorithms they use. If some of the citations to your article are not included, chances are that the citing articles are not accessible to Google search robots or are formatted in ways that make it difficult for their indexing algorithms to identify their bibliographic data or references.