Well, the point is that you could argue that for subjects such as Chemistry/FM there should be a larger % receiving the top grades as to even sit those A-levels the bar is higher in terms of needing 7-9s whereas much lower in some other subjects.
Also, it doesn't matter if you get a B in chemistry if your're looking for STEM courses but if you switch, e.g. to Business where someone doing General Studies (A*) will be taken instead of someone achieving a Chemistry A.