But overall these things will tend to average out and it is still a valid measurement because it is no doubt intended to be used as an indication of current queing time - as opposed to an absolute and precise measurement.
Not so sure about that, these timings are submitted to the CAA as the recorded queuing times for security. Hence the suspicions about data manipulation.
The point in relation to this thread is that when operatives tell pax that it is a legal requirement to do the 2nd scan, and/or that they will not be allowed to fly if they don't, they are making it up (whether deliberately or through ignorance).