Exercise causing hyperglycemia

Depends on the intensity of the exercise. As a rule, low intensity cardio lowers levels, higher intensity or competitive sport raises levels.

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Simple: Take extra insulin for any sport which raises your BG (start of with half the correction dose you'd otherwise take to correct the high).

The reason for this is that the "BG up = carbs eaten" idea is massive oversimplification; in this case, hormones released in response to intense exercise (adrenalin) cause your liver to dump more glucose.
The fluctuation might be caused by exercising at a different intensity, general stress (e.g. BG might go down during training but go up for league matches because of stress), etc.