I must confess that it is a long time since I last flew any of this series of aircraft but there was a time when I flew them regularly including instructing on them. From memory I wouldn't expect the slats to deploy much above 55 kts, certainly not when turning onto final at speeds close to 70 kts, why would they need to. The aircraft ASI should be checked in the first instance in my view. An over reading ASI is not uncommon.