Roger,
I suspect you already know the answer to your own question regarding why manufacturer's do what they do. Money has a major influence. If the bed in a finite sized van can be 'shrunk' it can make the rest of the van look more spacious and persuade gullible Joe Public to buy it. After all you can usually test drive a car, or even take it home overhight, but it is rare to be allowed to use a new van before purchase. I am one of those strange people who has been seen crawling over the beds in show vans with a measuring tape, and usually ending up departing muttering comments about 'midgets' etc. I was also amused to read some of the latest issues of the caravan press where the new Bailey Ranger fixed bed model gets prominence. However, the bed width in that van is 135 cm or 4ft 5inches in real money and then tapers, as do many towards the foot. My own van which has an indentical layout and outside dimensions, but is from Europe, manages to get a width of 145 cm to the bed, an increase to 4ft 9 inches. Even with the taper this gives both my wife and I sufficient room to sleep very comfortable without rolling into each other. The difference is displaced from the washroom. I spend at least 8 hours every day asleep so I feel that this time should be spent as confortably as possible, otherwise everything else in the day goes downhill. I usually only use the washroom a few short times a day, so no problem. So you takes your choice, and pays your money, and hopes it will be alright.