Quite often I find that when I move the Military Units to Bring slider up from 0, the % success drops a few points; moving it back to 0 doesn't restore the initial, higher value.
More generally, the exploration events tend to assume that the hero is part of a substantial expedition, even when no military units are taken. This is explainable (every expedition contains a number of civilians who don't get mentioned unless an event brings it up) but the impression formed by picking 0 on the slider is that the hero is just heading off into the woods alone. So this is one of those things that *feels* like it might be a bug to the player, even if it has a justification.