I think this is a minor bug.
There are two ways of setting alerts. The standard one (as suggested in the user manual) is to hold SET and then to select ALERTS. Here we can select Sunrise, Sunset, Storm and Hourly (https://www8.garmin.com/manuals/webhelp/instinct/EN-US/GUID-85CAA854-BC33-4D9F-B3E2-6142FFA418CF.html).
There is a second way of setting alerts. SETTINGS - SYSTEM - TIME - ALERTS. Here we can select just Sunrise, Sunset and Storm. However, if I turn Storm ON, then I trigger also (or just, I don't know) the Hourly alert.
Just try it yourself.
1. Hold SET, select Alerts and verify to have disabled the hourly alert (I have all alerts disabled except Storm in this menu).
2. From SETTINGS - SYSTEM - TIME - ALERTS, turn ON the Storm alert (it was off).
3. Then, manually set the time to - say - 10:59. After 1 minute (at 11.00), the hourly alert will start.
4. Disable the Storm Alert from SETTINGS - SYSTEM - TIME - ALERTS, and repeat point 3. The hourly alert will not start.
I tried this twice. My version is 2.60.
