Sandisk Extreme HD VIdeo low write rate.

I bought a Sandisk Extreme HD VIdeo recently which has an advertised up to 30MB/s write speed. I have formatted it wioth SD Formatter and tested the write speed with h2testw only to find the write speed is only 16MB/s. It was bought from a large reputable online supplier.

Am I using the wrong formatter? 

Do I need a specific USB adapter?

or is this correct?