Have a play with the PORTAMENTO settings. In z3ta hit the GL tab, just after the 6th oscillator. You have the settings in there. The portamento mode and time etc, also the pitch bend range if you on a midi keyboard.
I'm not entirely certain but I think it has to do with whether the notes overlap or not, like Stix mentioned. I've tried automating a pitch bend as well but it doesn't always sound that good, especially if the sound you're using has any lfo type mods on it.