There's a poem that was written, if I recall, right in the middle of the seventeenth century - by a Cavalier, no doubt - "Amarantha, sweet and fair, Oh braid no more thy shining hair". Something like that! And Anthea was also in use round about then as a literary name or nickname. So it wouldn't be much of a stretch for a family to choose reliable Samuel for a boy, trendy Aramantha for a girl, and then on her arrival to just merge the two ... .
Is Samantha an American invention which spread to England? Interesting if so!
I saw a totally ghastly movie once, a spinoff of GWTW crossed with Showboat, in which the Southern belle leading lady was Amantha. Not Samantha, not Amanda. She used to peer intently into the middle distance as if short-sighted, no doubt reading from the cue board. And that name failed to catch on!