It's a technique they use to fit the movie into the time period (and get in more commercials). They do often during the opening sequences, as well as the endings. It also seems like they might've sped up the sequence with Arceus reflecting back and seeing Ash. It could've been that speed to begin with, but I didn't see the version that aired in Australia.
I liked the song, and I admit, I tried to sing it, but I was way off because they sped it up, and I wasn't used to it.