I'm not an expert with respect to land titles so take my opinion with a grain of salt... However, I don't think the land title registry is the most authoritative source when it comes to boundary mapping. In my experience using it in the Brooks area I've discovered that certain lands belonging to the Eastern Irrigation District will be listed with the County of Newell despite being firmly within the boundaries of the City of Brooks. I had assumed this was because the some of the land in that title extended beyond the city, though it's also possible it's because the documents are out of date.... I'm not 100% sure. Either way, I wouldn't change the boundary based on that alone unless there was additional supporting evidence that this land is indeed part of the MD of Bighorn.