Thinking on it, especially since I started looking to try to document it, it seems to be a weird rounding error when fractional movement points are used (either because the terrain, such as hills or swamp, consumes one-and-one-half-movement points, or roads which moving upon consumes only one-third or so of a movement point). It isn't simply rounding up, though ... no matter how small of a fraction remains, you seem to actually be able to move at least one tile (so it should always be rounded up) but for some reason, an extra rounding up seems to occur sometimes, seemingly making half a movement point for some reason rounding up to 2.
