I suspect that is largely the result of Nicholas Meyer being the director of both Alley's Saavik in TWOK and Cattrall's Valeris in TUC. He seemed to prefer having the Vulcan characters show hints of emotion without really becoming emotional, whereas Nimoy seemed to prefer the totally cold, emotionless approach.It's doubly ironic that the Saavik expy in STVI, Valeris, is stated to be fully Vulcan but she's got far more in common with Alley's Saavik than with the Curtis interpretation.
The funny thing is that Nimoy the actor never played Spock as cold and emotionless. In fact, he once actually told Nick Meyer "I never played Spock as a man with no emotion. I played him as a man with deep emotions he was constantly struggling to keep in check." Or something to that effect.
Ultimately, it comes down to just two different directors. And I really don't have a problem with either interpretation of Saavik. However, if Kirstie Alley wasn't going to return and Nimoy was going to interpret the character completely differently, I think they should have just gone with a completely different character.