This is interesting and a feature that I didn't know about, and Hey Siri will often not trigger in the car for me while driving. Now I know to retry and I'll have a better chance at triggering Hey Siri.

"We compare the score with a threshold to decide whether to activate Siri. In fact the threshold is not a fixed value. We built in some flexibility to make it easier to activate Siri in difficult conditions while not significantly increasing the number of false activations. There is a primary, or normal threshold, and a lower threshold that does not normally trigger Siri. If the score exceeds the lower threshold but not the upper threshold, then it may be that we missed a genuine “Hey Siri” event. When the score is in this range, the system enters a more sensitive state for a few seconds, so that if the user repeats the phrase, even without making more effort, then Siri triggers. This second-chance mechanism improves the usability of the system significantly, without increasing the false alarm rate too much because it is only in this extra-sensitive state for a short time."