First of all, hat-banding can be caused by a number of things. The most obvious and easily preventable is to make sure you cut in the edges and roll the walls with paint that came out of the same bucket. This may sound obvious but in the rush to get things done often painters will open up 2 cans of paint, with one being use to cut in and the other for rolling. You can get away with this most of the time but eventually it will bite you. Assuming you follow proper painting procedure and do use the same bucket for cutting in and rolling, you can still occasionally run into a hat-banding issue. There is a phenomenon which happens sometimes during the colder months that I call temperature-related hat-banding. This can happen when a new house does not yet have the permanent heat source working so painters are forced to use portable heating units to heat the house while they paint.
If the painters arrive at the job site and turn on the portable heater, the temptation can be to start painting as soon as the air has warmed up. However, the walls may still be below 40 degrees. If this is the case, the paint will dry lighter than it is supposed to. Then by the time they roll the large areas of the wall, if the walls have warmed up above 50 degrees they will then dry to the proper color. All of the areas that were cut in with the brush will now be lighter than the rest of the wall. Since the areas near the ceiling tend to be more obvious and stand out, the wall will appear to have a light band around the top like a hat-band, thus the term for this phenomenom: hat-banding.
To prevent this from happening, allow a longer time for the walls to warm up when using a portable heating unit prior to painting a new home. There are digital thermometers available which will take a temp reading just by pointing it at a solid surface so you won't have to go on guesswork to insure that the walls are warm enough to paint. Happy Painting!