Grade boundaries exist to make sure there is consistency in exam results. If the majority of people find T2 difficult this time then the actual raw scores will be low. Maybe the last T2 paper was easier and people did better. So it's not fair on those just having completed T2 so they standardise the scores . 65% may have been required for a C in the last T2 paper bit it may be only 60% this time. At the end of the day the same numbers get A's , B's , C's etc each time