From The Old New Thing, a favorite blog of mine:
The makers of a certain major brand of detergent which I will not name (but which for the purposes of this discussion will be called "Snide") appears to take every step to exploit inattentive customers.
A box of Snide detergent powder comes with instructions indicating that for a normal-sized load, you should use 3/8 cup of detergent; for a large load, 1/2 cup.
The detergent box also comes with a handy measuring cup.
The measuring cup holds 5/8 cup of detergent.
Not to be outdone, Liquid Snide plays a similar trick.
The instructions indicate that for a normal-sized load, you should fill the cup to line 1 (the lowest line). For a large load, fill to line 2. If you look at the cup they provide, there is also a line 3 which is even higher than lines 1 and 2. Not even counting the imaginary "line 4" which is what you get if you fill the cup to the brim.
Just because it comes with a measuring cup doesn't mean you have to use it.