Being enrolled in an activity is not the same as being available or rostered onto an activity.
Being enrolled means that you are wanting (at some stage) to be a part of that activity. You can enrol in an activity when you join the activity's team. This will allow the activity manager to rostered you onto future activities.
It is up to you to look at the upcoming dates/times to select if you are available or unavailable. We find that many organisations will roster volunteers on if they have said they are 'available' or give 'no indication' of their availability.
To ensure you are not rostered on when you can't make it, put 'not available' next to the dates/times you are not free to volunteer.