They have a very accurate position at 07 due to the VHF ACARS transmission. They use that datum with the next ping to work out the return time at that known range. High school maths then allows you to work out future ranges based on ping return times.
But it doesn't make sense to take the GPS position at 1:07 for calibration, if for the next ping - which probably was exchanged at 1:11 - secondary radar position from 1:11 was available. Why then take the GPS fix from 4 minutes earlier and not the SR fix from that same minute ?