It certainly has nothing to do with the lack of tutoring as I know plenty of children who have been tutored over the years and not passed.
VR papers vary greatly in standard and GL assessment are easier than some . I did GL and Walsh with my Dd 3 years ago and she felt the real exam was closer in standard to walsh than GL. Also it is the final exam of the three and I think some children are beginning to flag by the end.
Interesting point regarding flagging - DS was at the delayed WHSB sitting and finished an hour later than should have well past normal lunch time, I wondered at the time whether that might have an effect. He also didn't have time for the last 15 questions other than to put down a random answer.