I asked the folks who run EconPapers (and RePec) and they said:
"Most likely just a difference in the different collections of “things” to search in."
"Numismatic is not an economic subject and Academia has papers from different areas, I think"
"SSRN and RePEc do not offer full text search. SSRN could, as they have the full texts. RePEc only links to them."
I'm not exactly sure what to make of Academia's results. I serached for "David Ricardo" (with quotes) and got 33,996 papers I can't see anything except the 84 papers with the search term in the title because I'm not a premium (i.e., paid) member. SSRN had 69 hits and EconPapers 542.
Not sure what any of this means.
Humberto Barreto