Well this is a worrying story.
Basically, due to marking inconsistencies, especially in a subjective subject like English, grades which fall close to a grade boundary are unreliable, because a different marker could have legitimately marked it differently.
The majority of students used to receive a grade A*-D, so 4 grade boundaries. Now that will be from 9-3, so 6 grade boundaries. As the number of grade boundaries increases, the number of students who fall close to a grade boundary and thus have an unreliable grade increases. This represents an increase from 30% of grades being unreliable to 45%.
The article suggests that grades should be scrapped, as the difference between a 79 and an 80 is slight, but one mark could get a B and the other an A, which will be viewed differently.
www.telegraph.co.uk/education/2017/08/20/thousands-students-set-receive-wrong-gcse-mark-new-system-experts/?utm_source=dlvr.it&utm_medium=twitter
Obviously the same problem will affect maths, but the marking is usually more objective.