it is a bit more complicated, beacuse the fan modifies its resistance when rotatin, it decreases with the turation, up to a limit.
if you consider it constant, in order that you have what you say means that when the fan is powered from 5 V it eats up 20mA, but i doubt it is so small.
thing of something in real life. try to find 2 lamps of the same voltage and different power or amps. if you connect them in series, you wil see that the one that has a smaller amps rating will make more light, because it has a larger resistance, and because they are passed by the same current then it will have a bigger voltage drop on it.
of course, lamps resistance modifies too, as they heat up. it can actually decrease 10 times from when it is not powered to when you turn it on.