20% more free

why is it that the conditioner bottle i have has 20% more product than my shampoo?

i've come to a conclusion - it's for people who were hardwired as i am. observe the dilemma:
i am coming to the end of my shampoo bottle but still have about 30% more conditioner left. i don't want to be wasteful, but i also have a really hard time using one brand or even specific kind of shampoo without it's nicely matching conditioner counterpart. what they want you to do is to then go out and buy shampoo thus making you a customer for life.

but i have fooled them.

i just keep the 30% left of conditioner and use it as shaving cream. either that or they just sit in my shower cluttering the sides ridiculously.