XbeeZigbee current profiles

I have observed the current profile (in picture it’s voltage across 2.5 ohm resistor) during whole cycle of sensing + sending + sleeping of waspmote, here I used preset delay of 3 sec required for reassociation. Now from the current profiles of broadcast & unicast transmissions I observed that the time for which radio module remains on is >3.2 sec (200 ms stabilization delays).

In case of broadcast the radio remains on for 4.3 sec consistently while for unicast radio on time varies between 3.5 sec to 5.6 sec.

So why does the radio on time differs by this much from 3.2 sec??

Why is radio on time in case of broadcast is consistent??

Why is radio on time in case of unicast is inconsistent??

What are the processes that take place during both broadcast & unicast transmissions ???

Here are the links to current profile pics

broadcast profile

unicast profile

Has to do with if the parent router is busy, if any node discovery overhead is needed and the time to get the ACK back where as broadcast just sends the message and that is it.