Cannabis is incredibly diverse, and here is why.
The physical characteristics (phenotype) of any organism is determined by the combination of their genetic makeup (genotype) and environmental influences that can cause changes in how that organism expresses their genes. This is especially important in plants since they can't get up and move when conditions are unfavorable, they have to change in order to grow where they are.
Generally speaking, there are two main categories of Cannabis- hemp and "marijuana"- which separate quite well genetically, presumably due to purposeful selection for different uses. Artificial selection by humans has reshaped the evolutionary trajectory of Cannabis into a species which arguably has no ancestral-type populations that have not been adulterated by cultivated Cannabis. Selecting and cultivating plants with desirable traits such as fiber content, seeds production, cannabinoid production, terpene profile etc. has diversified these groups even more.
Although there are expected differences between these two groups, Cannabis grown in one location can be completely different in another location, even when they are genetically identical. Cannabis is often propagated through cloning methods to minimize variation, but unless the environments are similar, the clones can grow into plants with unexpected differences. Differences in clones are not due to genetic differences, obviously, but rather are due to outside influences such as light, temperature, soil composition, nutrients, water availability, and more.