You have got to remember for Thorpe that even though MAP, TAP and Shark Hotel guests normally get in between 10 and 30 minutes before the park actually opens there are still engineer and ride operator checks with a manager or team leader have to sign the ride off. Only then the ride can open.
If an engineer hasn't handed the ride over to the rides department then the ride operators cannot do their checks and so on so ERT depends on whether the ride is ready and can open early. If not most rides are ready 5 minutes before the park opens.