Why would spending money on a tile lower my score?

Solution 1:

I have tried replicating this, with exact same conditions (map size, civ ect) but unless you play with some 3rd party mods, I will have to call this one a bug.

There was one version where the point system calculation was completely off. Can't remember which one.

Alternatively, someone might have messed with your GlobalDefines.xml, or if you were playing multiplayer, the game maker might have been messing with his GlobalDefines.xml. (You could do this in the early games and have messed up cheating / point calculation.)

But unless you get beaten militantly, it's rather hard to drop in score.