Here's why:
* Change in voice director: DiC Entertainment changed voice directors for Season 3. This can impact the overall sound of the show and lead to different vocal interpretations of the characters.
* Targeting a younger audience: The original DiC dub was criticized for being too mature for its intended audience. Season 3 aimed to appeal to a younger demographic, which could have led to Griffin using a higher-pitched, more "child-like" voice.
* Personal choice: It's also possible that Katie Griffin simply decided to try a different approach to her performance in Season 3.
The change in Usagi's voice is noticeable and often discussed by fans. Some find it jarring, while others simply see it as a different interpretation of the character. Regardless, it's a unique aspect of the DiC dub's history.
It's worth noting that the Viz Media dub (released in the 2010s), which is generally considered to be more faithful to the source material, features a consistent voice for Usagi throughout the entire series.