This is not something that we set directly; the system looks at the unique identifying number for each basic land card and takes the most recent one - specifically "most recently added to the database." Due to scheduling quirks, the HBG lands ended up being imported later than DMU, so they are still taking precedence.
It is certainly not our the desired behavior, but changing it would mean either changing the function itself (which isn't broken!), writing some kind of specific-case override (which is clunky and gross IMO), or modifying the card entries themselves (which could have any number of unexpected impacts on collections and decklists). Considering it is a purely cosmetic issue, it doesn't really rise to the level of warranting any of those actions. We'd rather keep pursuing a long-term solution to this issue.