When the Apple Watch first came out, it recorded your heart rate every 10 minutes, as advertised. After an update last week, however, users noticed that the readings became more sporadic. Now, Apple is responding to complaints by saying that missed readings are by design, not a bug in the watch's software.
An updated heart rate information page on Apple's website, first spotted by 9to5Mac, says that the device "attempts to measure your heart rate every 10 minutes, but won't record it when you're in motion or your arm is moving." The page was updated today, and it previously made no mention that certain movements would prevent a passive heart rate reading. (A separate feature, in the Workout app, can record your heart rate during runs or other physical activities.)
It's not clear why Apple changed how the watch functions. Owners started noticing a change in the frequency of heart rate recordings after updating to Watch OS 1.0.1, but the changelog does not say anything about a change in passive heart rate readings. Nevertheless, it seems most likely that Apple is trying to avoid inaccurately high readings caused by physical activity. If the passive heart rate readings are recorded only when you're seated or stationary, the values should be more consistent.