hi ,everyone

I am using IC610 and MMSIM13.1. I want to observe jitter of a PLL output. And I used eye diagram assistant in VIVA to process the PLL output .But I got two totally different eye diagram.one is obtained by edge triggered and the other jitter is small .But the other is large

I have already read VIVA's user guide part about difference between two ways to obtain eye diagram but still I do not get it.Could anyone help me figure it out or give me some paper to understand the difference of the two ways