It is probable that each ping is actually an exchange satellite to aircraft and back, or vice versa. This is the only way to ensure a secure identity check within each ping.
It is not clear which end initiates the communication. There are conflicting statements by experts.
In any case it does mean that each individual ping operation does get longer to conclude as the distance from satellite increases.
Along the published southerly path the distance from the satellite is increasing for the last several pings, at least. However, other possible paths can be matched as well.