Their programmed database & algorithms for things like blocking DDoS attacks, etc are updated real time on an ongoing basis. So, for example if at a given point of time if there is (example) a DDoS attack coming from certain set of (various) IP pools, locations, ISPs, etc then some real visitors also may get impacted, but only for the duration of that DDos attack. This is from understanding it from a technical point of view. But, overall the percentage of real visitors getting impacted is very very negligible (very very less), because their algorithms have a lot into it, that considers various aspects.