Having To Do With Diabetes: My Wife Takes Her Readings

Having to do with diabetes: My wife takes her readings twice a day. This morning she had a relatively low reading, 15i. This evening it was 3l5; My question is this. This huge difference was due largely because of what she ate during the day. is it not ?She insists it was because of what she had last week; she had some ice cream but I keep telling her that went thru her system Last week.