Short answer: YANBU
Long answer:
The 'goal posts' in English are not 'roughly the same' by any means. A new syllabus for English Language and Lit was introduced in 2010, meaning that although these aren't the first exams sat, this cohort is the first to get final grades under the 'new' system (which Gove has written off already!). Prior to 2010, the exam specs had changed very little for at least 5 years - so teachers have had to introduce, resource and teach a new spec. That's not necessarily a bad thing - change can be positive for everyone - but the implications of a move to a completely unknown spec with a different system of marking/grading must be obvious.
Without going into unnecessary detail, one of the big differences is this: marks used to be banded in 'notional grades', so the marks were divided into 8 sections, and when marking coursework or exams, you could see how the numerical mark translated into a grade. Now (certainly on 2 boards, I imagine it's the same for all) marks are in 5 bands, which do not correspond to grades. So you don't know what 'grade' you're giving when you mark. And you really don't. Schools have been told by boards to 'think in bands not grades'. Boards had consistently refused to give any information on where the notional grade boundaries might lie.
Most importantly, the band-grade correlation is NOT fixed. The boards are therefore at liberty to shift grade boundaries So (as seems to have happened here) the boundary for a C has been moved upwards by x marks, meaning that pupils who sat the exam (with the same markscheme) in January had to get fewer marks to get to the C grade.
So as the OP says, it's not harsher marking - the examiners themselves could have given exactly the same marks to an answer in Jan and June and it would have been a C in Jan and a D in Jun. It's the boundaries.
And I don't think it's fair at all - because it's allocating grades by percentage, not by the skills the individual candidate is displaying.