Using REW and Acoustic Timing Reference for Crossover measurements

A post by JohnPM (REW) on Audiosciencereview said USB mics can be used to measure individual drivers for crossover simulation if an acoustic timing reference and clock correction are used. I'm curious about the process. I tried it using two speakers: speaker to be measured on the Right channel and the acoustic reference speaker on the Left channel. I noticed that the clock update comment was different for each sweep even though I did not reposition either speaker. However, I am in a reflective environment. Should successful measurements show the same distance offset in each measurement when done properly?
 
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For example, in my reflective room I get

1st measurement
Delay -1.1867 ms (-407 mm, -(1 ft 4.03 in))
using estimated IR delay relative to Acoustic reference played from Master Volume L with 0.0655 ms (22 mm, 0.88 in) timing offset
Clock adjustment: -15.2 ppm

2nd measurement
Delay -1.0590 ms (-363 mm, -(1 ft 2.3 in))
using estimated IR delay relative to Acoustic reference played from Master Volume L with 0.0655 ms (22 mm, 0.88 in) timing offset

3rd measurement
Delay -1.1410 ms (-391 mm, -(1 ft 3.41 in))
using estimated IR delay relative to Acoustic reference played from Master Volume L with 0.0655 ms (22 mm, 0.88 in) timing offset
Clock adjustment: 14.5 ppm