Answer:
There can be many explanations to why a dandelion will show different heights, which is a phenotypical expression of genetic changes.
One of the most plausible, and which has been proven, comes from the theory of natural selection, explained by Charles Darwin. In essence, this theory states that any living organism will seek to adapt itself to the demands of the environment in order to survive and reproduce, meaning, pass its genes to the next generation. However, these genes must provide the necessary elements for that organism´s survival. If not, then the species dies and is replaced by those who did adapt to the environment.
When an organism, like this dandelion, is exposed to a certain environment, it needs to have the characteristics that will allow it to compete effectively for nutrients, water and sunlight. If the environment is a particularly harsh one, where these elements are not easy to get, then the flower will undergo specific genetic mutations to adapt to these needs, and these mutations will be expressed physically. Those that acquire the changes will survive in the new environment, and those that didn´t will die out.
The difference in height as an expression of genetic variation, therefore, is explained by this theory of natural selection and this ability by organisms to ensure their survival and reproduction, under certain circumstances.