Anyone feel free to correct me but this is my understanding of the sequence of time, going my memory.
The Snapchat photo of Abby was taken at 2:07pm
The video and the voice recorded by Libby was taken at appx 2:30pm.
According to GHs videos, the span of the bridge can be walked in about 5 minutes.
Shortly after 3pm, Libby did not answer her phone when her dad called, just before he arrived to pick them up.
So only my speculation, after the 2:07 photo the girls didn't immediately proceed onward to the SE end of the bridge. Maybe they'd never intended to walk the full length of it. The time sequence would suggest they were on the bridge for quite a few minutes, maybe just standing or sitting, enjoying the sun and the view, talking, or whatever girls do, after the Abby photo, and the suspect appeared later.