RE: Why Board Maker lie about actual volume ?
I don't think that they lie on purpose, but the marketing material is often created early on, before anyhow has had an opportunity to check the volume in a tank.
Another reason is of course that the developers trust that they are so experienced that tank testing isn't needed. And they just don't seem to learn...
I agree that it is really annoying when volumes don't match. This is especially true when we are talking about boards with a liter amount that is about you bodyweight in kg. Then 5 liters matters A LOT :-(