From the report (for those who couldn't be bothered to read it, but felt the need to express their indignation at what was or wasn't in the report nonetheless):
--Bold is mine--
"There are many rumours about subjects being regarded as 'hard' or 'soft' and different people will have differing opinions on the matter.
In general, subjects referred to as being 'hard' are more traditional and theoretical subjects, for example: English, History, Physics and Chemistry. In fact, all the 'facilitating subjects' listed earlier can be considered 'hard' with the addition of others such as Economics and Politics.
'Soft' subjects are usually subjects with a vocational or practical bias, for example: Media Studies, Art and Design, Photography and Business Studies. However, there are no set definition of a 'hard' and 'soft' subject.
Generally speaking, students who take one 'soft' subject as part of a wider portfolio of subjects do not experience any problems applying to a Russell Group university.
ATTENTION!! If you plan to take more than one perceived 'soft' subject, some caution may be needed."
Facilitating subjects list:
Mathematics and Further Maths
English
Physics
Biology
Chemistry
Geography
History
Languages
I think the key point is that this is not about what is difficult and what is easy. 'Hard' subjects (in the general definition given and through the examples) are generally those viewed as requiring an 'academic' study mode, i.e. 'theoretical' and/or 'traditional'. Soft subjects appear to be those which, using the language of the report, are slightly less 'theoretical' and are less of an 'academic' and more of an 'applied, rather than conceptual' nature.
I think the idea is that if you get high grades in 'theoretical' subjects you demonstrate the capacity for the study mode which is expected at the university level for most subjects. That is not a judgement on the relative difficulty or effort required for specific A Level subjects, simply a reflection of the specific capacities that universities, and specifically the Russell Group, are looking for. They don't value the fact that you got an A* in Chemistry less because you found it easier than you would have found Art, just like they don't value an A* in Art more because you found it harder than you would have found Physics. Where they value one more than the other, it is because they view it as demonstrating or developing a specific capacity that they are looking for.