A dense liquid such as certain acids is being loaded into a cargo tank. According to the AAMVA CDL Manual, why might the tank need to be loaded only partially rather than completely full?

AAMVA CDL Manual §8.2.7 (p. 8-8-2)

Overloading a tank with a heavy liquid can put you over legal weight limits even though the tank looks far from full by volume. Drivers need to calculate load amount by weight, not just by how much empty space is left in the tank.