My understanding is that there’s an enzyme coating the sensor that generates an electrical current in the presence of glucose. So the sensor detects differences in this current based on whether there is more or less glucose present in the interstistial fluid (this is fluid between cells), and it also uses changes in this current to decide whether your glucose is rising or falling. But it’s not actually measuring glucose nor measuring in mmol/L (or mg/dl) units, so it needs to do calculations to convert the data it gets into something that can be displayed and make sense to you.
So, for the first 24 hours of each sensor (and I’ve heard some say the first 72 hours, though I don’t know how accurate that is), your Dexcom is basically trying to figure out how to match the electrical current it’s receiving from the sensor to the blood sugar readings you’re providing. There is nothing inherent in the actual electrical current that says anothing about what blood sugar level it corresponds to, which is why you need to start each new sensor with two callibrations and make sure you callibrate regularly.
The glucose in interstistial fluid lags behind glucose in the blood by 15-20 minutes. So when you callibrate your Dexcom, you’re providing it with a reading that it “should reach” in the next 15-20 minutes, not with the reading that it should currently be showing (especially if you callibrate when there’s an up or down arrow). So, it nees to make further calculations so that it can adjust so that in 15-20 minutes the reading it displays is accurate accoridng to what your meter said.
At least this is my non-scientific understanding of how things work. It makes sense to me, at least, that the Dexcom would get “confused” and have to re-calculate a bit (and not be able to display things like arrows, or display the wrong arrows) after a callibration that tells it that in 15-20 minutes it should be much higher or lower than it had calculated. Kind of like when you suddenly go off the route your GPS planned onto a street it didn’t know existed, it has to pause and re-calculate everything based on the new information. Others can correct me if I’m wrong or provide more detailed/accurate information.