It's the opposite actually: they tried to replace certain Japan-only cars with West-only variants on the opponent list, but forgot to remove* the GT500KR that appears in the Grand Valley enduro, so every entry after it is pushed downwards.
Yep - in simple terms there are two copies of a certain opponent (the GT500KR), which breaks the numbering. There's one long list of possible opponents, and each event specifies particular cars from the list for its own entry list. Where the confusion probably came in is that each opponent has its own ID number, but the game actually ignores that and just looks for that position in the list.
As a basic example, imagine there are five opponent cars in a list:
ID 1 - GT500KR
ID 2 - Viper
ID 3 - Civic
ID 4 - Corsa
ID 5 - Focus
Let's also imagine that an event uses cars 3, 4, and 5 for some sort of cheap car race, the Civic, Corsa, and Focus.
However, when the GT500KR was duplicated, the list looks like this:
ID 1 - GT500KR (position 1 in the list)
ID 1 - GT500KR (position 2 in the list)
ID 2 - Viper (position 3 in the list)
ID 3 - Civic (position 4 in the list)
ID 4 - Corsa (position 5 in the list)
ID 5 - Focus (position 6 in the list)
Our imaginary event looks like it should still pick the Civic, Corsa, and Focus, but the game doesn't actually use the ID, it uses the position in the list. As a result, we would get the Viper, Civic, and Corsa as opponents instead.
To a developer at Polyphony, it would still look like the events used the right opponents (the entry list for each event didn't change, and the opponent IDs didn't change), but in fact the opponents after the GT500KR are now all moved downwards in the list, and the game code unintuitively only cares about position and not ID. Expand our list of 5 opponents to 1220 of them, and the GT500KR from position 1 to position 1127, and it's easy to see how it could be lost in there.
