Set formation is believed to be similar to what are called 'beats' in classical (linear) wave theory.
A fetch will generally produce a wave field, basically a lot of waves with different heights, different periods, and with different phase relationships. As the waves move out of the genesis region, many will start to combine. Roughly, toughs combining with toughs producing even deeper toughs, peaks combining with peaks producing even higher peaks, and toughs combining with peaks producing, actually they would tend to cancel. Of course, as waves are not only toughs and peaks, they're also everthing else in between, so everthing else in between will combine with everything else in between.
Beats occur when the periods of the combining waves have a particular relationship. The relationship is referred to as commensurable, it exists if over a number of cycles, at some point the the peaks or toughs of combining waves conicide, and it can be quite a few cycles before it might occur. What can result are areas where there are few peaks combining with peaks (seemingly flat areas), with apparently isolated regions were peaks seem to be combining with peaks, producing isolated regions where the waves are significantly bigger, i.e. the makings of a set.
A gale, though a fairly reliable wave machine, could hardly be described as consistent. They do not continuously throw out the same period wave. They can't even be counted on to throw out a wave in the same direction. Though synoptic reports might describe a gale with a few selected parameters, like wind speed, air pressure, etc. virtually any parameter used is going to vary throughout the gale 'region'. In the case of wind, intensity, direction and the stretch of water over which the wind is blowing (fetch) will vary thoughout the gale region. Also, its likely the gale as a whole is moving, so thats going to impact wave genesis too.
Another contributing factor is that (ocean) waves have a beginning and an end, that is they come into exsistence and then at some point whatever was making them stops making them. They'll also end when their energy is disapated in some manner, possibly by showing up at your favorite surf spot. They also move at different speeds, which is function of wave period in the deep ocean. So, in the very least, in order for waves to combine, they would have had to at least caught up to each other, so set formation is not likely to be consistent, i.e. set formation, like a gale, can not always be described by a single set of unchanging parameters - things vary.
That was fun, thanks.