This is only my opinion and how I see Dr.Drop fitting into this testings and any testing you conducting.
My loop is big and quite complicated consists of 3x GPUs, CPU, 4x D5 Pumps, 2x High flow USB meters, 8x 12TB WD Enterprise drives in which 2 of them are watercooled, they are at the back where no air is possible + 4x 480 Nemesis Radiators and ton of hard tubing.
Why I mention all this, because for me to take any of that apart for any reason is whole day of work and this system to be fully bleed it takes a month, even 2 in order to be as silent and to perform at its peek .
About 5 months ago I took 1 of the GPU blocks apart to see how well the Biocide works with Distilled water and weather I want to continue on using it, its been 2.5 years of constant use. I didn't wan to put the same block back on because I have 3 spare blocks, I always have extra parts in case i needed.
So I put that block on and assembled the loop, filled with water and as always, while filling with water I use external PSU so my system has no power, and what do you know! while the water was going through the system I had very small leak and the water was running at the back along the M/B.
Dr.Drop lets you test your blocks or your whole system prior to assembly or filling with water and I think that might be the main reason for that tool, there should be some allowable pressure drop tolerance so you not wondering if other factors casing the pressure drop but then again, the size and complexity of the loop will play role in readings as well.
That's what I was referring in my first response, if you run the same test with the same staring pressure to see if the pressure drop to the same reading