Why is 6 months road tax work out dearer than 1 year?

The government charge more for 6 months, therefore it's cheaper to buy tax for year. This kind of deal from the government is leading those who are struggling to go into, if not further into debt, either buying a year as it's cheaper in the long run but causes some financial problems for low income families. It should be exactly half. It's the low income families that can only afford 6 months that get penalised...by their own government! They don't charge more paying TV license in instalments.