My understanding was that dogs are used to give everyone pre-check style screening which is what makes the line move faster. Is this not the case?

Following one of the New York snow storms, I flew out of EWR. There was a really long line and I was at first disappointed that they did not separate pre-check at all, having everyone go through the dogs. However, everyone was given the pre-check type treatment after going through the dogs, so the line moved really quickly (and I did not notice anyone get the random beep).

At MDW last Thursday morning, the pre-check line was not open when I arrived (before 4 am). The ID checker told me that everyone would get pre-check style screening because we went through the dogs. After getting closer to the front of the line, I noticed that people started taking off their shoes and going through body scanners. I then moved to the next line where I heard a TSO yell at the line to disregard everything that they were just told and that they needed to take their shoes and belt off, take out all electronics larger than a phone, etc. I then noticed that the precheck line had opened and left to make use of it.

Upon going back to the entrance of the checkpoint, everyone (including those going to the pre-check line) was forced to go past the dogs. I could not understand why people in the regular lines were no longer receiving the pre-check type screening even though dogs were still being used.

Is it typical for dogs to be used but for passengers to still receive regular (not pre-check style) screening? Is it normal for the TSA to change the screening to this from pre-check style screening and to tell everyone to disregard everything that they were just told?