Why would the same payload have a slightly different looking IEEE 802.15.4 frame three days later?

Why would the same payload have a slightly different looking IEEE 802.15.4
frame three days later?

I have two xbee S1s that successfully communicate with each other. But I
am trying to get one of them to receive IEEE 802.15.4 frames from an Atmel
transceiver. The goal, of course, is to "trick" an xbee into thinking it
is talking to another xbee, when in reality it will be talking to the
Atmel transceiver. The only way, as far as I can tell, to do this is to
monitor the packets transmitted by an xbee using a sniffer and clone them
on the Atmel transceiver.
And here are my findings:
The packet sniffer shows the following traffic when entering a single
ASCII character "m" from an xbee S1:

The sniffer shows the following when sending a single ASCII character "k":

The sniffer shows the following when sending the same ASCII character "k"
two 3 days ago:

Aside from time stamps, why would the payload have "a*d*" one day and
"a*23" another day?
Would appreciate if the wireless network experts from the community could
weigh in.