Rucksacks. Litre size confusion

[Paul]

I'm looking for a new backpack.
Looking at a surplus site I found a nice little 37 litre bag.
I buy my milk UHT in cardboard cases of 12 x 1 litre bottles so looking at 3 crates, I'm thunking not too bad a size.
BUT, being cautious I check the physical size and on emailing got told the bag was 52 X 30 X 30 cms.
I'm no maths genius but capacity is Height X Width X Depth which comes to 46800 cu cms or 46.8 litres. (Lovely having convertors online isn't it.)

Advertised size 37 litres, and by their measurements its 46.8 litres???????

So I email and say something on the lines of either they've measured the wrong bag or the 37 litres is wrong. He promises to email back AND when he does the supplier has checked another bag out. "You're right" they say.
Same maths, different answer to advertised size or in this case 100 litre bag, 135 litres by calculation.
Can't understand why, never noticed this before.

Luckily it's the right way round i.e. more space than advertised so I would'nt have been short changed but now were both confused.
So my question to you experts is:-

How big is a rucksack litre?