Acoustic timing reference for driver time alignement in REW

I'm trying to physically place two drivers on a baffle so that they are time-aligned. To do this, i have first made a best guess, built a baffle with a certain offset, and measured each driver using acoustic timing reference in REW. The issue is that neither phase nor calculated IR offset changes at all if i move the baffle several cm towards the mic. It looks like all i'm getting is the minimum phase.

Is this a limitation of the acoustic reference-method? I'm using a separat speaker as reference.
 
Today it just worked. I get accuracy within a few mm, and ok consistency.

In pure desperation yesterday i ordered an xlr mic and audio interace so that i can use proper loopback. Can i expect the same or better performance when it comes to timing?