My guess would be with pancreas action in the mix, it will be hard to ever make 100% sense of the #s. Good cycles could be when his pancreas is really working it and vice versa, who really knows.
Bix had dental a few months ago (needed badly it turns out, though no sign of infection though he went on preventive ABs just in case). I was really hoping I would see his #s stay in DDs after that, but no go. I haven't noticed any change really - his pancreas was working before the dental, and seems to be at the same pace now. Oh well. But he really needed it and it turned out had a couple of the ROLs (or whatever they are called) that are very painful. His #s actually went up quite a bit a day or so after the dental and I had to shoot higher for a week or so, which freaked me out, but then after that he returned to about where he was before the dental.
Of course ECID, and if it's infection that causes higher BGs (vs. just cr*ppy teeth with no infection) that might explain why some cats get a lot of improvement after dental and others don't.
Just a thought off the top of my head here, no idea if this is medically sound or not, but what about trying a course of ABs (not sure how you'd get them, this is truly a "just popped in my head" random idea here) and see if his #s improve with that? If they do, then at least you would know that? Not sure if that's a good idea or a rotten (haha) one... if so, just pretend I never suggested it. haha_smiley