Why does the required outage percentage differ from one liquid to another?

49 CFR 383.119(i) (operating practices and procedures not otherwise specified); AAMVA CDL Manual §8.2.6, p.8-8-2.

Because outage percentages are liquid-specific, a tanker driver cannot rely on one rule of thumb for every load; they must know the correct outage figure for whatever product they're hauling, which is exactly the kind of applied knowledge the endorsement test checks for.