I listened to the interview later, and don't think I heard it all, but he seemed to me to by trying to differentiate cases of consensual sex between an adult and an underage girl from what he termed more "serious" cases, ie non-consensual sex between adults. That seems reasonable enough to me, and if uunderage sex figures count towards the statistics overall, then it is bound to bring down the average tariff accordingly.
Victoria Derbyshire was, I think, too quick to jump on the 5-year average tariff figure to illustrate her assertion that "rapists will be back on the streets after a year and a half" when there was no evidence to back this up.
I think that it was a poor interview, and Clarke was ill-advised to use such an emotive topic to illustrate what was intended to be an attempt at saving costs, and that he should have been better-informed. However, I do not think he should resign, and he has already apologised (presumably under a great deal of pressure).
And isn't it the case that ALL convictions are subject to a reduction in term length for early guilty pleas?