Where did the chorus vocals of Sonic Generations' Open Your Heart come from?

I watched piplupfan77’s excellent video on the different versions of Open Your Heart from Sonic Adventure. In it, he mentioned a remix found in Sonic Generations. He observed something about its chorus:

in Sonic Generations […] it also sounds like some of the vocals were re-recorded for the chorus.

The chorus vocals were lower in pitch than the original song and followed a different melody. Where did they come from?

Despite what he said, I doubted that the vocals were re-recorded. The rest of the remix used the same vocals as the original song, so it was likely the chorus had been reused too. Besides, the original song was over a decade older than the remix. If the singer had re-recorded the chorus, the difference in his voice would likely have been noticeable.

I searched for an answer and found nothing. Then I returned to the original song, and I realised where the vocals were from. They were part of the original song! The original chorus had a vocal harmony that was a fifth lower than the main vocal melody. It’d been there ever since the very first version of the song, back in Sonic Adventure and its soundtrack. The “re-recorded” chorus vocals had been there all along, and I only noticed them once I looked for them.