I wouldn’t be surprised if this is a phenomenon that has been well-documented for people who study LCHFs in athletes.
I know they’ve shown that basically, after a certain amount of time on a high-fat, low-carb diet, the body changes how many of the enzymes it’s producing for primary conversion of carbs into ATP. The body ramps up its production of enzymes needed for lipolysis energy generation, but since fat metabolism is basically carb metabolism with a few extra steps tacked on, the enzymatic reaction to produce energy still proceeds more slowly than if someone were burning primarily carbs. In other words, your VO2max decreases, which is why LCHF is not a great diet for people who are doing workouts at all-out intensity for very long. (LCHF is fine for an ultramarathon, but pretty crummy for a marathon if you’re hoping for a competitive time)
This is total speculation, but I’m guessing that if you’re changing how well adapted your cell is for burning fat, you may also be changing the receptors on your cells that ferry in glucose. If the chemical reaction going on inside the cell is proceeding more slowly, then you might expect a lower internal cellular glucose concentration leads to a “closed” state where GLUT-insulin receptors would be less responsive to insulin. Or there would might simply be fewer of these glucose transporter receptors over time if you’ve been low-carb for a while. THIS BY THE WAY IS A TOTAL GUESS!
I do know that when we met a person who was ultra-low carb with a 4.9 A1C, she said half a glucose tab spiked her 70 to 100 points. My son spikes about 40 points from 1 glucose tab and he’s 2!