Whoever decided that the term "Continual Improvement" should be part of the ISO9001 standard probably didn't realise the drama that it would cause. If they did they probably would have chosen a different term.
The difference between the two is that continual occurs over a long period of time but with intervals of interruptions where as continuous occurs over a long period of time uninterrupted.
It is due to this "same same but different" issue that many people building a system for the first time will use the terms interchangeably. Just like people use the terms "certification" and "accreditation" interchangeably.
At the end of the day, if you just use "continual" you won't have any issues. In my early days building systems I was given a corrective action for using the word continuous instead of continual in a quality policy. As far as I am concerned that is just a prime example of an auditor being a jerk as opposed to a genuine fault in the system requiring in depth root cause analysis. In this case my root cause analysis determined that my highschool English teacher failed to cover the complexities of continual vs continuous. My corrective action could only be to go back to highschool and do English all over again.
I bet I'm not the only one that has had grief over these words.