As Tenacity mentioned, part of it could be the direction - I don't know for sure if it's the case, but it could be that Meyer and Nimoy wanted the acresses to portray Saavik differently to suit their own visions of the character. Equally, it could be the actresses' own interpretations of the character - being the second person to play Saavik, Curtis may have weanted to put her own spin on it rather than just doing an impression of Alley's Saavik.
And as Donago and Bry_Sinclair said, in TWOK she was originally meant to be half-Romulan, so this is how Alley played her, though all mention of it was removed from the film. And in TSFS, this aspect of the character was completely removed, so Curtis played her a bit more straight and 'Vulcan-like'. Honestly I don't have a problem with it, though I'm fairly forgiving of discontinuity within Star Trek (honestly, I think you kind of have to be else you'd drive yourself crazy). I did think her "David is dead" line was delivered very well and showed that maybe she was struggling to keep her emotions in check.