Has anyone had this happen?
I wake up and Dexcom wants a blood sugar. Dexcom says my blood sugar is 140. I take my blood sugar with my meter and it says, 140. So rare that I match Dexcom. I put in 140 and then my receiver says, 129. Why does it do that? It finally was right on the money and then it switches to another number.
Thanks for any explanations ahead of time. Laura
Yes, mine does this all the time. It's supposed to be following a trend as to where you're sort of headed - and I also think it's for Dexcom's safety, frankly. It does drive me crazy though!
It just adapts the result of its calibration function. A perfect match is always nice to see, but doesn't necessarily reflect reality. In this case, unless you had been perfectly stable at 140 and were going to stay at that value for the next 15 minutes or so, your interstitial glucose was lower than your BG if you were on a rise, higher if you were on a drop.
Very interesting!!!
I was on a RISE!
Keep in mind the Dexcom does not give an instant BG reading. It's more of a trend or average of BG over the last 20 minutes. So what you can see by entering the BG test you did was that over the past 20 minutes, and that you BG average was 129, not 140. Basically you're in the middle of a rise in BG. You'll see the Dexcom to go 140 or near in the next 20 minutes after that correction entry.
Without going into to much details, the Dexcom tries to adjust what it displays based on what it measures and on a linear fit calculated on the six last calibrations (that was the case before the Artifical Pancreas firmware update, not 100% sure of what it does now). That's why you may see the Dexcom trend higher, or trend lower than your BG at times. Since six calibration points are involved, one single calibration will not necessarily change the displayed value much, or in the expected direction. Here's a very schematic example of what that can lead to: a correction below the value could actually cause future measures to be too high in some circumstances 
It's of course a bit more complicated than that in reality and, in practice, there is no need to worry about it as long as the Dexcom operates within its specifications.
Interstitial fluid glucose (CGM sensor glucose) lags fingestick glucose by about 15 minutes. So if your meter calibration was 140 and your Dex had previously detected a rising trend, then it would make sense that it displayed a 129 after calibration. If it worked like a textbook case, the Dex would have displayed around 140 about 15 minutes later.
It'll be nice when CGM technology becomes sophisticated enough to factor in the interstital/capillary BG difference and actually predict/display the expected BG value.
Yes, this is about what the Dexcom lag is. In reality, that varies a bit from body location to body location, and the real physiological delay is shorter (around 6-7 mins). In some parts of Europe, we have other devices (the Freestyle Libre) that track faster than the Dexcom does. On the other hand, the Dexcom AP algorithm is supposed to be faster, but not available in Europe yet. I am currently comparing the two http://type1tennis.blogspot.be/2014/11/dexcom-g4-non-ap-vs-freestyle-libre-vs.html - 
I have seen the same thing but after a short time, its' accuracy improves to where it was. But not always! My big gripe is how much it seems to drift overnight. I think there is some sort of magic in the device.
I have been having Dex problems lately after two years of almost flawless results. A bunch of sensors were replaced and I’ve had a couple of conversations with the same Dex Tech. He told me that if my Dex result is close to my meter result, I do not have to calibrate. He said that sometimes the Dex needs to do something when you calibrate and that’s why you might see it jump to a number that doesn’t make sense.
The problem with not calibrating is that you keep seeing the blood drop. But for now I am following his advice.
FWIW, my 2014-8 (exp 2014-8-12) batch has been a bit flakier than usual.