GTX 970 bandwidth calculation


I am trying to calculate the theoretical bandwidth of gtx970

As per the specs given in:-

Memory clock is 7Gb/s
Memory bus width = 256
Bandwidth = 72562/8 // *2 because it is a DDR
= 448 GB/s

However, in the specs it is given as 224GB/s

Why is there a factor 2 difference? Am i making a mistake, if so please correct me.

I am new to the forum, I am not sure if this is a right place to ask. Pardon me if it’s not a right place and please redirect me to the correct forum.