I have CKD. My GFR was 28 and it is 35 now. How can that be if you can't improve kidney function?

I have CKD stage 4.I have been as low as 15 and at one point popped back to stage 3. In Feb 2020 I was back to 26. Why the changes? It's rare, but I think I have a problem with statins. The first time I was on Crestor I had fallen to stage 4 while waiting to see a specialist and the last thing I tried was stopping the statin. I had been complaining about a strange pain in the Middle of my upper right arm for about a year. I started to wonder about a brief item I had read years before about the San Francisco earthquake and that it was the first time they had studied a condition where crushed muscles died and released toxins that destroyed the kidneys. There was this one sentence that said they had recently seen it on very rare occasions with stations. No way right? I stop the statin and with in a couple of months I'm stage 3. I am thinking it's a fluke and since my cholesterol is still high we switch to very low dose of Lipitor. My numbers start to go down but don't think it's anything more than the natural slide to failure with stage4. It was the pain by Aug of 2019 that made me try stopping the statin again. It had spread in my right arm down to the wrist and up into my neck. When it started in th upper left arm I realized I couldn't afford to loose the us of both arms and tried stopping the statin again. A 22 in Sept and a 26 in Feb. Although I haven't been back since because of COVID I have felt so good I have a feeling I won't be lower, in fact I may be higher. All the pain has not totally gone down, but it's manageable now. Shortly I will have the kidney function tests again so I may have to update this. Also, the decision to place me on a state was completely the correct protocol. This problem is very rare and I must still find a way to lower that cholesterol which can be a killer in it's own way.

I am at Stage 3 and my GFR also varies.The explanation I was given is the GFR can be affected by other things (ie GFR is determined by Creatinine level and Creatinine level can be dependent on diet, hydration etc. Also some meds can affect GFR.
A personal observation... a couple years ago I received a pacemaker and my GFR has improved since then. I guess increased kidney perfusion has had a positive effect on kidneys (I had had a very slow heart rate before the Pacemaker).
So, from my experiences, GFR can vary. What may be more important is how much protein and creatinine you are spilling in your urine.