This is something that, typically, is only known by botanists or studious gardeners, that the flowers or sexual parts of plants are what is used to determine how closely or distantly plants are related to each other. In other words, it matters less how closely plants resemble each other in characteristics such as leaves, than it does in their flowers. Illustrated in the top photo are a mixture of succulents known as Jovibarbas and Sempervivums. They can be easily mistaken, one for the other, since their habit of growth and leaves are so similar. However, when it comes to their flowers, they do stand apart, as illustrated in this bottom picture.