Venom, my theory is that there is a hierarchy among many runners on the subject of greetings.
Many runners will acknowledge their equals or betters, but won't condescend to acknowledging runners who are lesser mortals than themselves.
So, if this theory is correct, then, for example, slow fat slobs can expect to be ignored when they try to say "hello" to many of the faster, sleeker runners. The sleeker faster runners will however acknowledge other runners who are on parity with them in terms of ability and prowess.
Perhaps you could run this model through on your next sample run, and let us have the data from your further research...