Because my brain just keeps on churning, and since I rewatched Lesson Zero and Luna Eclipsed, I noticed a few things.
First of all, Princess Celestia does have the TRADITIONAL CANTERLOT VOICE, and we've already heard her use it once. When? In Lesson Zero. When Celestia appears just after sunset, she yells "TWILIGHT SPARKLE!" That's not her normal voice. It's the "You are in so much trouble that I had to talk in all caps" voice. Also, building on that, why didn't Celestia arrive sooner? Because she had a sun to set, of course. Notice how she appears exactly as the sun is down. Twilight is important, but the sun comes first.
Second of all, take note of the music that plays as Princess Luna's chariot streaks through the sky. Recognize it? Try listening to the music that plays when Twilight faces off against Nightmare Moon and they charge at each other in the second pilot episode. It's the same theme. Luna and Nightmare Moon share a theme, which makes sense.
Finally, a somewhat subdued d'aww on Luna's part. Notice her expression right after Pip says "You're my favorite princess, ever!" Remember why Luna turned evil, at least according to history? I get the feeling nopony ever said that to her before. She looks stunned speechless. It also explains her outburst right after Twilight snaps her out of it. Also, she even gets dramatic lightning at happy outbursts. Wonder if she does that on purpose?