At last I have a reply from KLM:
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Dear Mr Dave
We would like to inform that on all flights of duration over 2 hours and 30 minutes a meal is offered.
When the system says "no meal", it means that you did not make any special request regarding meals.
Should you actually wish to make that type of request, your Flying Blue team remains at your disposal either by telephone on (+ 44) 0870 24 29 242 from the United Kingdom or by fax on (+33) 1 58 68 68 00.
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So that's all right then, though it does not explain why the return flights had a meal detailed.
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