Are A-level Grades from different Examination Boards of equal merit?

At Maths A-level (including Further Maths) I have tutored pretty much all the modules across the major examination boards (Edexcel, AQA, OCR (and MEI variant)).

Whilst the material covered for the most common modules (C1, C2, C3, C4, M1, M2, S1, S2) is broadly similar I find the degree of difficulty in the texts and exams varies quite significantly between the exam boards. Broadly speaking Edexcel is the easiest and OCR (MEI) the hardest.

Which begs the question: are the grade boundaries varied to compensate or do we have a situation where the merit of Grades varies depending on the Examination Board a school uses?