So, I was curious, and I've peek into the decompiled code.
It was found that there is no difference between this two messages. It just picks one at random to show:
Game1.showGlobalMessage(Game1.content.LoadString(random.NextDouble() < 0.5 ? "Strings\\Locations:Mines_Infested" : "Strings\\Locations:Mines_Overrun"));
