I remember this issue being brought up before, as I had experienced it myself, but unfortunately I don't remember if there were any definitive conclusions made on the topic then. I don't remember a solution. Like Alain said, the onset phase of the osc's seems like a highly probable cause. But if I'm not mistaken, I think the amp envelope(s) was also being discussed as a possible reason for this behaviour, ie the "jump" from wherever (what level) each voice env is at, to wherever the starting point of the env is when a voice is retriggered. If my memory is right, I think I've had this issue even when not even using unison, just a lot less noticable then, as even with chords the notes are rarely triggered at the exact same time.
Wish I had more information and less speculation on the topic.