Fantastic and fantastical

Solution 1:

Dictionary.com is oversimplifying in this case, but only by a little. The OED draws a clear distinction between the two words, despite many senses that directly overlap.

All of the non-obsolete senses of "fantastical" have equivalent senses under the headword "fantastic." But, as you posited in the question, there is one primary non-obsolete sense of "fantastic" that does not correlate with "fantastical."

A.7. In trivial use: excellent, good beyond expectation. colloq.

That's it, if we only examine senses that are in modern use. Archaic uses of each word are more expansive, but still, almost every definition of "fantastical" is cross-referenced to "fantastic" in the OED. Dictionary.com probably decided that the words were similar enough to be considered variants of the same headword.