The space between groups of packets is heartrate, not encoded in any way at all: one group of three pulses per heartbeat. So this device is very simple indeed: each time it detects a beat, it transmits one group of three pulses. I say this just because one of the measures group intervals (I think there was only one such measurement shown) matches a reasonable heartrate, and also because simpler is better, couldn't be simpler that this.
I suggest that one of the pulses is the marker for beat, and the distance to the others is device ID (temporary ID, it seems).
The reason the device has to be away from the heartbeat so long to start transmitting a new ID is practical: it's to avoid the device and the receiver having to find each again, in event the device loses the heartbeat. This happened to me a lot when cycling with a chestband that had lost its elasticity, and kept sliding out of position.