I have type 1 and if I remember correctly, meters are allowed to be off by 15%. So if you 77 was 15% low, that’s 88. And if your 89 was 15% high, that’s 75. So these numbers are not actually that much different at all.
I am a non-diabetic who monitors!
Did you check it at a different time than usual? For example, blood glucose is often high when you first wake up in the morning. That’s a normal, well-known phenomenon. Your body pumps extra glycogen out of storage to help you wake up. I have seen mine as high as 110 mg/dL if I check first thing upon waking, even though I’ve been fasting 12 hours at that point.
Or did you have any changes in physical activity? Anything that might have caused your body to release stored glycogen from your liver or muscles to use as fuel?
It could also simply be that the meter isn’t perfect. Since you immediately tested another finger and got 10 points lower, I would suspect some random error in the meter happened for whatever reason. Maybe something was on the first finger that slightly contaminated the sample, or who knows what happened.