A message may be transmitted multiple times, until it's received without errors. This can happen in every network. If you use a simpler protocol, that e.g. only sends a pulse, no such delays can occur. Travel speed of RF is almost constant, else GPS couldn't work.
You could add a wired connection between sender and receiver, where the sender signals the start of a transmission, so that you can find out whether there is a real and varying delay between the message transmission and reception.