A prospective cohort study follows 10,000 postmenopausal women over 5 years to examine the association between hormone replacement therapy (HRT) use and incident ischemic stroke. Among all women, the relative risk (RR) of stroke for HRT users versus nonusers is 1.5 (95% CI 1.2–1.8). When stratified by hypertension status, hypertensive women have an RR of 2.0 (95% CI 1.5–2.7), while normotensive women have an RR of 1.1 (95% CI 0.9–1.4). Which of the following best explains these findings?