I wonder whether another possible reason might be that someone ordered it, but was then fast asleep by the time the mains were served. I've certainly fallen asleep during that interval - particularly if orders are taken really early in the flight - and I've also seen others do it.Slightly differently, after reading here about how meal choices are done, I now try proactively to tell the crew that I'm not going to have any dinner at all if my plan is to go straight to sleep and not eat. CC explanations here made my realise why, on one flight, a cabin crew member was loitering outside the toilet just waiting for me to come out so that she could take my meal order, when I'd actually been cleaning my teeth and getting changed into what I was going to sleep in.