Database files are already in existence so the research and development has already taken place. Therefore the power improvements are already coded into the modified file and cannot be altered. Because the mod file is designed to work with the engine not necessarily the vehicle it’s in, there is extra safety built into the file. (BHP output is the same as a custom file, but how long that power stays in, is limited). 

Best way to describe it is “flash and go”. You get a noticeable improvement, but you could get more out of the engine and still be within the safe limits with a Custom Remap File.