That which is continual is that which is either always going on or recurs at short intervals and never comes to an end; that which is continuous is that in which there is no break between the beginning and the end.
My research on the suffixes (below) motivate my conjecture: The Latin etymon of -ous denoted fullness, and so 'continuous' meant fullness of continuity, and so meant no interruptions.
However, the Latin etymon of -al lacked this denotation of fullness, and so the possibility of interruptions was ascribed to 'continual'.
How sound is my conjecture? I ask because I beware of the Etymological Fallacy.
-al Etymology 1 (of 2)
From Latin adjective suffix -ālis, or French, Middle French and Old French -el, -al. [...]