Sorry, deleted my comment because I didn’t think anyone saw it yet. Looking closer this appears to be a consequence of them changing the names of the original oblivion. Looks like it messed with SteamDB. Checking the page for the deluxe edition shows it was always $20 (which SteamDB is wrongly interpreting as the price for the base game).

So in this case Bethesda did the appropriate thing and tried to clarify the original is the 2009 edition and I rushed to get upset at them for something they didn’t do.

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