I realize this is an old thread, but I haven't been here for awhile. In case the op is still following, I have a very similar situation to share:
Similar to the op, I live a very low carb lifestyle. My A1C has crept up to 5.7, which has always been normal prior to the diet changes 5 years ago. This seems to be a common occurrence with otherwise healthy low carbers. This has attracted the attention of some doctors whom are now studying it. Some theories are adaptive glucose sparing, or that red blood cells are living longer than 3 months, so they accumulate more glucose. Another common occurrence with the healthy, insulin sensitive low carbers is extremely low fasting insulin levels. Whatever the reason is, I don't sweat it anymore. I used to test my glucose often, but not so much anymore because it is also consistent. The conversion of 5.7 A1C to average glucose of 117 seems off to me considering my glucose monitoring. My glucose stays steady around 95ish between meals, and 105ish after meals. I I am insulin sensitive with very low fasting insulin. For now, I just politely listen to my doctors when they tell me to eat less sugar and carbs to get my A1C down. It is literally impossible to cut any more sugar and carbs down than I already have.