Its just the way that the tiles are numbered. If you note, the maps are (broadly) rectangular rather than square. If you get into the editor, and turn on "grid coordinates" under the view menu, you will see how the tiles are numbered. They go from (0,0) to (99,99) for a standard map, but there are only odd-odd and even-even pairings (eg. there is a tile marked (95,99) (odd,odd) but not (96,99) (even, odd).
Not 100% sure why they do this....Might be something to do with the geometry formula's they use for calculating internal distances.