It looks like Garmin is just following the AHA’s guidance, as noted in the text beneath the graph in the app.
Based on data from the American Heart Association
The AHA recognizes hypotension, but they don’t publish a color coded chart for it because low BP is symptom driven and varies a lot by individual. They only provide a visual chart for the hypertension categories. If Garmin created its own low‑BP band, they’d be defining a threshold the AHA doesn’t chart, which could bring liability concerns. Sticking to the AHA hypertension chart avoids that.
