Surprised to see my forerunner 235 register a pretty authentic heart rate when it's on my handlebars?? Is it estimating based on previous data from speed/cadence etc? I can't find any mention of synthesising heart rate data when it can't detected a heart rate. The optical sensor proves itself when on my wrist, but it is obviously not getting data off the rubber Garmin handlebar mount. No other HRM connected by any other means