840 Solar giving me false new maximum heart rates and not using my manually entered resting heart rate - using some value almost 30 points higher. Where is this coming from?

Newly discovered, seemingly unusual maximum heart rate values have been detected by various Garmin devices such as Edge, Forerunner, and Fenix wearables. Numerous threads in corresponding forums have reported these findings. Personally, I have experienced similar "issues" with the Garmin 840, where my maximum heart rate is automatically set well above my usual maximum heart rate recorded during activities. However, it appears that this discrepancy is the result of a calculation performed by Garmin to determine your maximum heart rate without requiring you to reach it (which can be quite demanding, actually). Whether this calculation is accurate or not remains uncertain. What I would appreciate is an official communication from Garmin that explains how this calculation works. While I don't expect them to divulge patent-protected details, a high-level explanation would greatly increase trust in the displayed values.