The runaway-speculative part of my brain, the part that likes a good spy thriller, has come up with the idea that they may be sending unique files at certain times, with the sheer number of individual data transmissions obfuscating that that's occurring. But in real life there's no reason to suspect that, because any time I decide to fire up DIGTRX and check the files, they're the same ones to the best of my ability to tell. (With all the seemingly random characters encrypting whatever message they're sending, it's pretty difficult to ascertain with certainty, but any random bits that I compare always match up.) Also, the sheer incompetence often on display with HM01 makes it unlikely that they could try to do such a thing without risking it going sideways because of some operational error at the transmitter site. And finally I imagine that there are people in American counterintelligence with the thankless task of monitoring all HM01 transmissions, so nothing would escape them.
I think Retops' hypothesis is perfectly reasonable. HM01 broadcasts are a total waste of energy and only adds to the wear and tear of their equipment if it's not serving some purpose, and that sounds as good of an explanation as I've heard.