In another question in EL&U "Positives changes on the cards" — meaning? ,
it came up that at least one of us AmE speakers had always heard this idiom as "in the cards" and never as "on the cards", whereas at least one BrE speaker had always heard it as "on the cards", never as "in the cards".
However, Ngram searches show both forms in literature from both sides of the pond, with "in the cards" clearly in the lead (since before "on the cards" occurs, up to the present); it is now about 20x more common in BrE corpus, and about 100x more common in AmE corpus.
However, "on the cards" does not seem to appear in AmE corpus before c. 1850, and not before c. 1825 in BrE corpus.
Answers to a related question favor the Tarot explanation for both. One quotes a source claiming Charles Dickens as first documented use. However, looking at quotations of Charles Dickens I have found, so far, only literal meanings, referring to playing cards used in gambling, not Tarot, and not using the idiom in the sense we know it now.
I suspect the origin of the two varying phrases might differ. Could someone please explain how, and from where, these two similar idioms arose? Can anyone prove that one form is indeed just a variation on the other?