I read somewhere that AMD's Turbo Core functions similarly to Intel's Turbo Boost, then the only way to get to 2.8GHz is if only one core is being used and if the temperature does not go beyond a certain point. There are like different Turbo Core speeds when two, three and all four CPU cores are being used. It could be that 2.4 GHz is the max the APU will go when 3 or 4 threads are being used.