Why is that the write speed of DDR SDRAM are faster than the DDR2

Hello, I have two different types of hardware, one with DDR SDRAM and another DDR2, i tested them with bw_mem tool for write bandwidths, seems that at higher writes of >512kbytes the DDR is faster than the DDR2(more than 40%). But when compared to reads, DDR is slower (more than 50%). I couldn't find any reference that explains why. AFAIK, the DDR2 works double the faster rate than the DDR. any ideas? Thanks,