Hey beyond, you - or someone, at any rate - has posted this same basic question at least once before, in a slightly different context. I thought it was a good question then and still think that. I frequently have the same thoughts while reading this board, and also while looking - for example - at the lengths used when magazines do ski tests. (It appears in many cases that a single length is tested by magazine testers who come in a very wide range of sizes, and that single length - when reported - is typically very much on the long side, unless it's specifically a test of women's skis.) Setting aside the special case of real slalom skis, I can think of several possible reasons for this phenomenon:
Manufacturers would rather their skis be criticized on line or in the press for being too powerful than for being too wimpy, so erring on the long side when providing skis for testing is preferable. To the extent that skis for testing are provided by their manufacturers, ski testing is as much a marketing activity as it is an analytical activity.
Testers and reviewers (male testers and reviewers, anyway, who appear to be the majority) would rather be seen as being on a longer ski than a shorter ski, to the extent that ski length is thought to reflect ability, experience, personal strength, and general tendency to take after John Wayne in all the things that matter to the male ego.
While the marketing piece aimed at getting people to buy a ski aims at long ski lengths for sales reasons, manufacturers' sizing schemes presumably reflect their desire - and their retailers' desires - for the skier who buys the product actually to LIKE the experience of skiing on it. Therefore these sizing schemes reflect the reality of how big people actually are, how well they actually ski, and where they actually ski, rather the reflecting their fantasies.
Much of the image - and, to a lesser extent, the reality - of American skiing centers on the West, where spaces are bigger and more open, favoring longer boards. Much formal testing seems to occur in the West. You only have to look to this board to see that reflected, at Keelty's site, and also in magazine tests, etc., etc. I would be interested to hear, however, what proportion of ski sales occur in the East vs. the West.
Note that a common thread here is the general bias on the part of American culture - and American men's culture, especially - to privilege things that are big and and to denigrate things that are small. Paging Dr. Freud. I suspect that male competitive psychology plays a huge part in these kinds of recommendations. (Please note that I don't mean to pick on wasatchback here at all, personally.) Everyone has seen the kinds of testosterone-driven posturing that happen on this site from time to time (or more often than that). Why should ski size recommendations be any different?
I actually make a point to recommend shorter than what I buy skis to alot of people. In fact when I moved to stowe, my quiver instantly shrunk 10 cm all around for the most part. Now that I out at the bird for a couple weeks my shorter quiver is more demanding than the 190-195ish skis my peers out here ride. Its easier to maintain balance on longer skis, especially in chopped, bumped snow at speed in low light. My 183 Katana although a really sweet ski and would probably be the right ski for most people my size, is not up to the task of 5 turn cirque runs or stomping airs in choppy snow. Most people arent looking for a or need ski that can do that but I am the minority that needs it.