I came up with a possible answer to my own question. If I remember correctly, when I bought the GTP a few years ago, the DIC was broken. So I went to the junkyard and replaced it. But maybe the DIC from the junkyard was not from a GTP. So perhaps there's a chip in the non GTP DIC that thinks it's in a car with smaller fuel injectors (for an L36) than the fuel injectors on the L67? If so, then it's calculating fuel consumption based on L36 squirts instead of L67?