Hangerhead, the explanation you got was probably true although I understand your suspicion. The summer season is full of slots which means that flights get delayed and crews run out of hours. In a low cost airline, any delay picked up will be nigh on in impossible to catch up later as the planes pretty much fly "back to back", meaning that when one rotation is done, a crew will be waiting to take over the plane for a departure half an hour later. If the first flight of the day is delayed by an hour, you can do the math with sector 8 for that plane on the day with subsequent slot delays etc.
When management run crew numbers, they assume everything runs like clockwork and as a low cost company, any buffer is seen as waste. Penny wise, pound fool. This all happened in 2006 too.