When I think of a NAS, I generally think of it in terms of a storage device. Then again, that kind of ignores the whole “network-attached” part!
That may seem like a minor point, but not when it comes to measuring data and transfer speeds. Confusing storage bytes with networking bits doesn’t just cause confusion, it might have implications for what is actually possible with your NAS. Tony Bourke put together an excellent primer to straighten everything out.