We don’t know for sure that the accused turned Samantha’s phone off.
He may have turned his off but not hers, or both or neither.
For all we know, Samantha’s phone may have been zipped up in her leggings (or the like) and detected later. We heard about the 5pm Buninyong ping very early on but nothing more. What if her phone was on the whole time?
In Toyah Cordingley’s current trial, the jury heard that accused person Singh's phone didn’t connect to any towers between 1.16pm and 8pm that day but Cordingley's phone could be tracked moving south. This suggests that Singh turned his phone off, and that Toyah’s phone was driven away from the crime scene whilst still on. I think Adrian Bayley also drove away from the crime scene with Jill Meagher’s phone on in his car IIRC.