Just like meters can have a 20% variance so can the number on your CGM compared to your meter. So if your bg is 100 by meter then your cgm could read anywhere from 80-120 and be considered accurate. Since it is a percentage then the higher the bg the higher the allowed variance. At 200 meter reading you could see anywhere from 160-240. Also, since a cgm is testing interstitial fluid, not blood, it may lag behind as much as 15-20 minutes from you actual current blood sugar reading. The body sends glucose into the blood first so that it gets to your vital organs quickly. It isn't in as big a hurry to dump glucose into your interstitial fluid and keep a few fat cells alive. That is why there is a lag. CGMs are intended to be a guide used to see the direction and rate of rise and fall so you know how to react. Also, the first day of a new sensor can have wacky readings that should settle down after 24 hours.
With all that said my dexcom is accurate most of the time. Not to bash any manufacturer, but the  dexcom is known to be more accurate and user friendly than the medtronic cgm. This is why you see many medtronic pumpers using a dexcom rather than the integrated enlite. Take the time to study and learn exactly how the device works. Learn how to adjust the settings yourself so you aren't dependent on a medical professional. I strongly suggest finding an online group dedicated to your specific pump and cgm. I use dexcom facebook groups and they taught me so much about how to get the most accurate results and prolong the life of my sensors. We all love this group and our dear friends here, but nobody here is using your device. Don't give up the fight, hopefully you will learn more and be able to get benefits from your cgm. I love my dexcom so much and can't imagine being without it. Good luck to you.