Secret Jardin int 300 and int 150

I am planning on using either one int 300 or two int 150, when looking at the website it says dimensions are 300X360X242 and volume is 19.4 cubic meter, and dimensions for the int 150 is 150X360X242 and volume is 9.7 cubic meter. Now I don’t think they would calculate the volume wrongly but would like to know why the difference because if I am to calculate the volume I’d get 26.13 and 13.07 cubic meter respectively

regards

any info pls? I emailed them but I didn't get any reply