By seperating grades into bands you allow for variety between examiners. A students final english exam might be valued at 71% by one examiner or 74% by another but they still get the same grade (a B1) and so the same standing when viewed by colleges.
But thats not what is meant by A levels here I believe. I think people are actually talking about the difference in state examinations. In Ireland we have our Junior certificate (taken at about the age of 15) examined on 9+ subjects usually, and our leaving certificate (taken at ages 17+ usually) examined on 6+ subjects. The A level is the English (and I believe northern Irish) equivalent to the leaving certificate but examined on typically fewer subjects but with greater detail on those subjects.
The leaving cert is a joke because it fuels an arbitrary “points” system for entering colleges, and the A-levels are under fire because too many people are getting top grades and therefore not differentiating between students of greater and lesser ability.