How are whiskers in a Boxplot of different lengths? [duplicate]

The formula for calculating the position of the whiskers is

Q3+1.5IQR and Q1-1.5IQR

But doesn't that mean that the length of the whiskers should be of the same length? How are they of different lengths in some boxplots?

Not necessarily. The whiskers actually end at the highest point within $Q3+1.5R$ and at the lowest point above $Q1-1.5R$. So for instance if $Q3+1.5R=100$ and the highest value in your sample is $90$ then the whisker will end at $90$. You must be observing something like this.