Why might a driver only partially fill a tank when hauling a dense liquid such as some acids?

49 CFR 383.119(i) (operating practices and procedures not otherwise specified); AAMVA CDL Manual §8.2.7, p.8-8-2.

Weight and volume are not the same thing in tanker work: a tank that looks only partly full can already be at the legal weight limit if the liquid is dense enough. Drivers who load tanks to a fixed volume without checking the liquid's density risk an overweight violation.