A couple of things which may be contributing to this
- the rate at which your high carb meals are digested. I occasionally have a chocolate binge and eat one of those large bars of Aero or something similar. That is about 60g carbs. If I have my usual insulin ratio dose, I hypo after about an hour and then my BG goes sky high a couple of hours later. If I have a normal size bar, I don't get the same affect. I have learnt to manage this by splitting my BG ... when I can't resist the chocolate fix.
- the time at which you eat your high carb meals. Since I have been on the pump, I have realised my insulin needs vary even more than I thought throughout the day. This is not just my basal dose but also my I:C ratio changes depending on the time of day so, if you are having high carb meals at different times to your lower carbs meals, the higher carbs may be exaggerating the different I:C ratios.
Sorry, my response is all about Me, Me, Me but I hope my experience provides some insight into what is happening with your body.