huh. I always thought that subtile was an affected misspelling. the OED online has this curious note:
In the 1st Folio of Shakespeare the instances are about equally divided between the spellings subtle and subtil(e, -ill. In the first editions of Milton's poems the spelling suttle (with suttlety, suttly) is the only one, except in Paradise Regained, which has subtle (with subtilty).

so I guess along with suttle (and others), it's yet another variant.