The amount of the increased risk has been misunderstood by many women. This increase is actually an increase in relative risk. There are a number of reasons. The main one is that the increased risk is actually very small, and studies can be influenced by many other factors that can cause different results to be seen. For example, maybe women with a late menopause take estrogen more frequently than women with an earlier menopause.
Late menopause is known to be associated with a higher risk of breast cancer.
In this case, late menopause and not estrogen increases the risk. One factor that could increase cancer risk is weight gain. If the risk of breast cancer with estrogens were high, all studies would show this easily, but studies have failed to reach a definite conclusion. Thus, experts have pooled the information from a large number of studies and analyzed the information. This type of approach is called a meta-analysis, which means that the information from all studies is added together.