S6 CPU never pass 1.5 GHz with low benchmark score

i just bought new S6 G920F with 64 GB and stock 5.0.2 android
i found my score lower than in reviews and in other users in Antutu 5 both 32& 64 bit get 57k while it should be 65+ k ,geekbench 3 get around 4400 while it should be 5200 accordong to gsmarena....

any way i updated to 5.1.1 with same results and i found my CPU never get higher than 1.5 GHz :(

whats wrong ??