Most "generic" tunes are designed for a broad range of year models spanning many different PCM
codes. This limits the number of parameters and tables that are modified, because many different PCM
tables have variances or are not compatible with other PCM
versions or families. So in the end, there isn't as much fine tuning that goes into the programming. In addition, most generic tunes are rarely updated or changed over time.
Custom tunes are tunes that are written for each specific PCM
code. Typically more tables are modified and fine tuned, and you get a smoother driving truck. However, that's always subject to the tastes and driving style of the truck owner. All custom tunes have their ups and downs, and there's no such thing as the perfect tune. Most people, such as yourself, can see the difference when more time and effort is placed on creating a high quality tune for your truck. The tuners who develop these custom tunes are constantly hard at work to improve their products and make changes based on customer requests and/or to adapt to knew products or technology.
The biggest difference simply comes down to time, effort, and listening to customers.