I think its something to do with body temperature and the type of exercise!
The other day I went for a 2 hour gentle stroll, not getting sweaty or anything, and just before my evening meal I tested 3.8
Yet the previous week went for really hard walk got all hot and bothered and tested 12.8, but did stop halfway for a cup of tea and a toasted teacake

. I have noticed that short vigorous exercise seems to raise my BG, now whether that's my body releasing glucose when I start to exercise and me not doing enough to burn it off or something else I don't know.
Perhaps others could do some research on this as I have noted others saying the same thing.