one old age pension for everyone

in 2016 the government introduced a two tier state pension scheme where new pensioner (april 2016) would receive a flat pension of £150 per week whilst pensioners (pre 2016)would receive their existing state pension of £125 per week surely if all pensioners have paid their national insurance contributions all state pensioners shold receive equal pensions why should some new pensioners receive £100 per month more than existing pensioners( pre april 2016) its not at all fair