I think the answer is down to productivity. Under the Tories, productivity failed to grow at all. Many put this down to lack of investment under what was often termed 'austerity'. Instead, the Tories were happy to import huge numbers of immigrants. Under Labour, there are tentative signs that productivity is now increasing. But paradoxically, this means that fewer employees are needed and a small increase in unemployment can arise in the short term whilst the economy adapts.